Dear 100 Hour Board,
From what I understand, adultery used to be illegal in at least parts of the U.S. I'm assuming that's largely because of religious influence, but were/are there non-religious arguments for it?
There are a variety of reasons for laws criminalizing adultery, many of which are indeed rooted in religious morality. However, there are also other logical reasons that are rooted largely in sexism.
On one hand, you could make the argument that criminalizing adultery would protect women, who relied on their husbands for financial security and had relatively few protections under the law. Punishing men who cheated on their wives could help limit the effect of wives and children left bereft of the traditional family structure and thus without a way to support themselves.
However, considering the long history of sexism, protecting women doesn't seem like the most persuasive argument. More likely, these laws were enacted in order to protect men. For instance, inheritance laws of past centuries usually left wealth and property to a couple's eldest son. Lacking the scientific knowledge for DNA testing and the like, criminalizing adultery could help ensure paternity. It also would allow a husband better grounds for divorce if he suspected or had evidence of adultery. In times when a wife was viewed more as property than a partner, it would also help protect the husband's investment in his wife by discouraging infidelity.
Personally, I think there's actually a decent, gender-equitable argument for making adultery civilly illegal. (Not going to go into criminal legality). Specifically, a legal marriage is a relationship willingly entered by consenting parties, generally with an understanding that certain conduct (including, as relevant here, divorce) will not occur within the relationship. Violation of those expectations is fundamentally opposed to the relationship, and often results in divorce, which often results in both concrete and intangible harm to the wronged party. Summary? I think you should be able to sue someone for committing adultery against you if they agreed not to. Basically a breach of contract. So, at least when it comes to civil liability, there's a good argument to me for making the person who violated the "contract" liable for the reasonably foreseeable damages (for example, the costs of both parties' attorneys in the divorce) resulting from the harm.