I don't think masturbation is cheating, I think however it can become a problem as you are saying it is robbing you and him of sharing a healthy sex life.

Being bisexual doesn't mean you are cheating either, I"m bisexual abut I would never stray on my partner, I am completly faithful to whoever I am in a realtionship with. Sure if I'm in a relatioship with a man I may look at another woman and find her attracitve or if I'm in a relationship with another woman and look at a man and find him attracitve doesn't count as cheating.

What I consider cheating is the mindful and deliberate search for another person without your partner's okay, wether it be a sexual realtion with another person than your partner, or a emotional connnection (no I"m not talking about friendships, I'm talking about carrying on a deep emotional relationship with someone that is similar to what you share with your partner)

But everyone's definiton of cheating is diffrent, what matters is like a few others have mentioned you seek help about any problems your having in your marriage or partnership with a therapist. Sometimes one doesn't see eye to eye with their partner and needs a neutral third party to help partners see on the same level or realize that what they are doing is harmful to the relationship.

Best wishes to you Bebop
Typo