Dear 100 Hour Board,
If a guy has an erection while kissing his girlfriend, is that bad? good? a sin? not a sin?
No, it is not bad or sinful. And for that matter, SEXUALITY IS NOT EVIL.
Seriously. Sexuality is a part of being human. Breaking the Law of Chastity is evil. Sexuality and having a fully functional reproductive system are not evil.
Having an erection is a normal part of being male. It can be sparked by any stimulus, be it physical, visual, audio, olfactory, or completely imagined, that is remotely related in any way to sex. It can even happen spontaneously. Erections are an unconscious response to stimuli and have no bearing whatsoever on the morality of an action.
I suppose it's wrong for a guy to have an erection if he's trying to make it happen. Otherwise, it's a totally neutral thing. All it means is that he's capable of being sexually aroused, which, again, is not evil. That is it.