Op, I work with about 40-50 gay men. I think a few identify as bisexual but choose to sleep with or date mostly / only men. Some joke that they said they were bisexual until they were brave enough to say they were gay. More crudely some call male bisexuality "gay lite". Their term, not mine. I am not saying this to be goady but just to give insight into non-PC language used sometimes in the gay community to talk about how they have experience the development of sexual orientation.
I think those who are saying that you are accusing people who identify as bisexual of being more likely to cheat, are missing the point.
You aren't saying he was more likely to cheat because there are more people to cheat with, proportionally. Rather, that you worry that his curiosity about a very real part of his sexuality may come to the fore in the future more strongly than it is at the moment. I don't think this is entirely inaccurate...
I may be biased because I work in the field and have lots and lots of friends / colleagues / clients who are gay men, but I would really see if you can find ways to facilitate him exploring this before you get married, if there is a way of doing so which is comfortable to you.
I love that he wants to reassure you and believe that he believes it. I just worry that in future as life progresses and he settles into himself more with age, this may come up again.
Then again- it might never come up and I could be totally, totally off base.