+2, cheating is cheating. You have mental cheating, like you may have on a chat room, and of course physical. If your doing something that you wouldn't do in front of your SO/ spouse, it's cheating.
BUT, if a female is cheating with another female, why not just ask your SO/spouse to join. Chances are he's all for it....
Definitely cheating, unless it's some weird situation where your significant other is in favor AND is involved. And that's about as healthy for a relationship as drinking pure sugar water is healthy for a diabetic.