I saw a couple people saying "this is cheating" or "that's cheating" in the strip club thread, and then the discussion of Tom and Eve in CW's thread came up, and so now I'm really curious about what's considered cheating in your relationships. I know it's a different threshold for different couples and for different individuals, and I'm legitimately FASCINATED by it.
For me and FI, it's like this. FI's bi, so if/when he decides he needs some boy bits in his life, he knows he's free to get them so long as he's upfront with me about it (preferably a day or so notice). I don't have boy bits, and I've got no desire to go get any more permanent ones than I can buy from Adam and Eve or whatever. So this is easier.
I'm not bi, but I am curious enough that I'm down for threesomes in theory. In practice, I may find out that I'm not able to handle them in a healthy way, but I want to try. So, obviously a threesome is likely going to have a person of the opposite gender to one of us, and we're both cool with that.
Outside of any potential threesomes, sexual contact with a person of the opposite gender is cheating. If at any point emotional connections that go beyond friendship are formed, no matter if there's a sexual component to the friendship or not, that's a problem. Getting his boy-bit fix without telling me beforehand is cheating (and vice versa, should I discover that lady-bits are a thing I like).