I personally don't see it as cheating. When I write a character, it is just that, a character. It isn't me in any shape or form. My characters take on a life of their own, and most of them do not even come close to my personality in RL. Some people seek out erotic RP for sexual gratification, while others seek out more of a creative outlet. I myself do not write for sexual gratification. I love to write, to explore new worlds thought up by myself and my writing partner, the fact that there are some hot steamy sex scenes in those stories is a bonus. To me it's not much different than picking up a delicious novel and reading it, only this novel I'm helping to create.
Now if you're talking about cybering, I still don't see that as cheating. Granted every couple is different, but for me personally, cheating would involve actual physical contact. Like if I were to meet someone I met online and actually have sex with them, then yes that would be cheating, but chatting online through a messenger of some sort, if you break it down, it's all words on a screen. There's no 'real' contact, no touching no talking....just words on a screen. To me that could hardly be considered cheating.
But like I said, that's just my view on it. Each person is different, so they have to analyze what they feel and view and go from there.
(and yes in case you were wondering, I am married, and yes my husband does know I write on E)