If your being deceptive, of course its cheating. My fiancee is bi, and we've discussed the issue. I don't have a problem if she wants to fuck around with another girl for fun as long as A. she is respectful to me in the process ex: doesn't lie to me, and doesn't deny my existance by telling the girl shes single. And B. her relationship with her girl toy does not negatively impact our relationship with each other. Other than that, it really has nothing to do with me. She frequently brings a girl home for both of us. As long as theres an understanding and no one is being misled or hurt, its all good.