I think that a lot of people think of bisexual people as wanting sex with partners of both sexes on a continuing basis, and it is hard to see it in terms of monogamy, maybe that's an area you could discuss with him sometime when things are calm and see if that's what is bothering him.

Finding out that you cannot provide some of your partners sexual enjoyments no matter how hard you try is a difficult thing to live up to also, it takes a pretty secure person to think that they can be enough to make up for that lack.