I agree with cara, rocket, Joe and the rest of you lot taking the stance here. I think that the person most culpable is the person who is in a relationship and cheats. However, a person who knowingly sleeps with (or even makes out with, blows, feels up, this isn't just limited to sex imo) a married or otherwise committed person is guilty. Sexuality has nothing to do with this conversation.
In the case of @Trapezeo
I don't care if that guy is a closeted gay male stuck in a heterosexual relationship, it is reallyshitty to cheat on your spouse. The decent thing to do as that man is to tell his spouse, and she would either be okay with him experimenting or not, and then they could decide to continue a mono relationship or break up.
What is the accessory to cheating called? The cheatee? Let's go with it. Those of you thinking the cheated is absolved from consequence, how would you feel if you were in a long time, committed relationship, and your best friend decides to sleep with your SO? That would be a dick move by your best friend, right? So why would it not be a dick move of a stranger who knows you are in a committed relationship? Help me see how you justify this to yourself.
None of this matters if the spouse knows about and is okay with it, but that isn't considered cheating, it is swinging.
To my knowledge, this has all been about sexual cheating, and we haven't delved into emotional cheating. But in abusive relationships, I don't think cheating is ok still, I do however absolve the abused from emotional cheating, and almost encourage it, if it helps them take steps to getting out of the abusive relationship. But that is a tougher topic I would have to think about more.