It seems like maybe we've got a bit of a definition problem here. Specifically, it seems like you're assuming that lusting after someone is the same (or really really close) to being turned on by someone, and that's not the case.
So what is lust? An Ensign article from 2016 clarifies that:
We tend to think of lust primarily as having inappropriate, intense feelings of physical attraction toward another person, but it is possible to lust after or covet just about anything: money, property, objects, and, of course, other people (see Topical Guide, “Lust”).
Lust compels a person to seek to acquire something that is contrary to God’s will. It encompasses any feeling or desire that causes an individual to focus on worldly possessions or selfish practices—personal interests, desires, passions, and appetites—rather than keeping the commandments of God.
Is it possible to lust after someone you're married to? Sure. If you start to look at your spouse as someone that you should manipulate or coerce so that you can have sex with them because that's what you physically desire right now and you don't really care about anything else, then you may be lusting even in a relationship that permits sex.
However, the fact that you have physical passions and appetites is not the same as lust.
This Eternal Marriage student manual clarifies (Quoting President Kimball) that "'In the context of lawful marriage, the intimacy of sexual relations is right and divinely approved. There is nothing unholy or degrading about sexuality itself, for by that means men and women join in a process of creation and in an expression of love." (emphasis added). The President Kimball quote goes on to clarify that even in marriage, sex is neither solely for having kids but nor is it "indiscriminate." A quote from President Kimball (when he was Elder Kimball) later in the same chapter clarifies that "Even though sex can be an important and satisfactory part of married life, we must remember that life is not designed just for sex. Even marriage does not make proper certain extremes in sexual indulgence."
Elder Holland said in conference in 2010 that "Love comes with open hands and open heart; lust comes with only an open appetite."
So, while I'm not going to try to get into a list of all the ways you could theoretically sin sexually in marriage (because that's not my job or my place), here are a few things that are clear to me:
1) You are supposed to be physically attracted to your spouse. (Note: by extension, you are most likely going to be physically attracted to them when you're still dating. Being attracted to them is good, and we just need to be sure that we act and monitor ourselves to ensure that we wait until it's appropriate to act on our attraction.)
2) A significant purpose of being turned on by your spouse is so that you will want to have sex because sex serves important purposes both in procreation and in relationship building/maintenance. (Going back to dating: one reason you are physically attracted to people is so you will want to get married to them).
3) It is still possible to use a spouse for gratification or as a sexual object, so even in marriage we need to pay attention to the way we view and relate with our partners so that sex is for a pair of people showing love to each other and not just for one person who wants physical intimacy right now.
So, to answer your questions more clearly:
What I’m trying to ask is there a clear boundary between appropriate God given feelings towards a spouse and lusting after them.... I... think that you could have base desires that could ruin a marriage.
There is a boundary, and we can evaluate it in different circumstances we come across.
Basically, we should be reflective and thoughtful in our relationships. Alma 12:14 clarifies that we can be condemned by words, thoughts, or works (actions). So, I think that gives us a few different ways to monitor sexual relationships.
1) Words: Is one partner trying to verbally pressure a spouse into something, or are they mutually expressing love, affection, and appreciation? (Note: I think that appreciation of a physical body is an appropriate part of this, for the record. It's fine that a spouse appreciates the fact that their partner's body is a beautiful and attractive creation; what's not fine is if they see their partner's body as just a tool for making them feel good.)
2) Thoughts: Is one partner thinking about ways to get what they want to please themselves, or are the two both in a mental state of wanting to bring each other (as well as themselves) joy and love?
3) Actions: Is one partner physically acting in a way that makes the other partner feel degraded or unhappy, or are the two acting in a way that brings them closer together?
A lot of these are going to be very context dependent, so it's hard to lay out a hard-and-fast rule beyond watching what you're doing, thinking, and saying when regarding physical intimacy. Fortunately, once baptized we have the right to have the Spirit with us, which helps enable us to recognize when we are and are not acting in accordance with God's desires, even in situations where we don't have an exactly-applicable black text rule.
If a guy I know watches p0rn and then gets extremely aroused and that’s a terrible sin. But when a guy sees his wife naked he has the literal same chemical rush of dopamine/lust. I know they’re in a committed relationship, I’m just worried it feels the exact same for the man.
First off, it's not necessarily the same: for more information on the science of porn and the brain, see the resources of Fight the New Drug, an anti-pornography organization.
Moving past that, though, I'll acknowledge that both situations can result in being turned on. However, the fact that there's physical similarity in reaction is not the point. If I feel happy when I key my jerky co-worker's car but also feel happy when I read the scriptures, the problem is not that happiness is a bad feeling. The problem is that I am seeking that feeling in a way that isn't appropriate. I shouldn't avoid happiness by not reading my scriptures any more, I should stop keying people's cars.
I'll state again that God gave men and women sexual desires for each other because He wants us to have them for use when circumstances are appropriate. Sex is physical, and that's good.
I think it's helpful to look at the context of porn vs. sex in marriage, rather than just the fact that people can be turned on in both cases. In pornography use, the focus is that someone feels the desire to achieve a physical feeling and they're willing to objectify another person (almost always a stranger) with whom they have no relationship at all, solely for the purpose of feeling physically good. In marital intimacy, the focus is that two married people (who probably do still want the physical positives of sex for themselves) are seeking to bring happiness to each other and to relate in a way that shows, feels, and strengthens their love (and maybe will produce a kid, depending).
So when I get married should I prevent anything hot or steamy from happening to prevent lust. I just don’t want anything unnatural to happen in the future. I’m worried if just all men are just perpetually lustful beings and if they just view they’re wife as ‘an acceptable’ form of p0rn.
To address your second line here: a spouse is not an acceptable form of porn, porn is a perversion of what is acceptable and beautiful between a wife and a husband. The thing that's the original, intended form here is the spousal relationship, not the porn. I'll even go one further and say that if we allow our perspective of sex to be twisted so that we can't enjoy it as a part of our bond with our spouse, we're letting Satan control the way we view a blessing God has given us, and we don't want that.
When you get married, your spouse should probably be attracted to you (there is, I believe, a small contingent of people who marry without any physical attraction present, but I don't think this is a huge group or something that anyone needs to actively strive for. If you find someone you love so much in other ways you're willing to forgo the physical attraction side of things, that's just fine! But not being physically attracted to someone shouldn't be a GOAL for most people).
Sometimes one of you will find something "hot or steamy" when the other doesn't and you'll just act like you would when you were single: you'll be like "Oh, huh, I'm turned on by that. Well, not the right time so let's go focus on something else." Maybe that'll happen for one of you more often than the other, and it won't always be the guy - it's perfectly possible for the woman in a relationship to be the one with a higher libido (or for these things to shift over time). If that's the case, she's the one who will more frequently redirect her attention or energies away from something that both partners don't currently want.
Please note that while we should be respectful of our spouses and appropriate in our own actions, it is the responsibility of each of us to avoid lust for ourselves. A wife does not "make her husband lust" by changing in their bedroom while he's there, or by bending over the oven to take out a casserole and thereby drawing his attention to her butt, or whatever. The husband in each of those situations determines whether he lusts after his wife by objectifying her body for his selfish desires or whether he loves by appreciating and admiring his wife in ways that appropriately strengthen their bond.
I’m sorry if I come across so jaded or crass, I just know so many women who’s lives have been torn apart by lustful men.
I'm sorry that women you know have suffered from this; no person should have to, and it is an unfortunate truth that our society's understanding of sex (especially as it is marketed to men, but increasingly to women as well) is really messed up.
However, I want to point out that you probably (hopefully) also know a ton of women (and men) whose lives have been blessed by partners with whom they have sexual relationships that are a beautiful, positive, and sacred part of their marriage. You just don't hear about these people because most people don't comment in Church or chat in casual conversation about how great their intimate life with their partner is (particularly in Latter-day Saint culture, which would urge modesty on such things). However, just because people don't always talk about this doesn't mean that they can't ever. You may find it helpful to have private, respectful conversations about your concerns with close family or friends in healthy relationships, or to read books or other materials published by the Church (like the Eternal Marriage manual) or by educated individuals who know about both sex and the gospel (see, e.g. the book And They Were Not Ashamed). They may be able to provide a counterpoint to some of your concerns.
It's good to learn from those whose experiences provide examples of what to avoid and warnings of how to be careful. It's also important to make sure that you date (and marry) someone who loves and respects you. However, it's also great (and for many people, important) to date people who are attracted to you and to whom you are attracted, because that's a great part about being married and can deepen the ways in which your relationship will be able to bring you together. There are ways we can mess up just about anything in our lives, and that includes a physical relationship with a spouse, but just like we can use the Spirit and watch what we're doing/saying/feeling in other areas of life to ensure we stay on the right track, we can do that with intimacy as well.