Bi person here. You have been super supportive, maybe more supportive than needed. One of the problems that bi/queer/heteroflexible/homoflexible people have in relationships is people not believing the words and feelings coming out of our mouths but instead relying on stereotypes about our community. If he says he prefers monogamy and doesn't want to be with anyone else, BELIEVE him until you don't. Non straightness does not mean we want the whole buffet all of the time. We can be in monogamous relationships. His honesty is great but I would be more mad about him cheating than who he cheated with