If your partner doesn't know and consent to it, then doesn't matter the gender involved, it's cheating.
Having said that, I do feel for people who are bi without their spouse knowing (and being able to accept it) and how that would put even more stress on doing the right thing by staying faithful. Sort of damned if you do and damned if you don't. Tell the truth and they feel betrayed, act out the truth and they are betrayed, keep it all to yourself and you feel betrayed by yourself. Tough place to be, but still, not a license to cheat.