If he enjoys sex with the OP, he isn't gay.
He might be bi - depends on your definition. Is any woman who has fantasized about being with another woman bi? Or who kissed on at a party when drunk?
I don't think the labels matter in the slightest. What matters is:
Is he happy being intimate with the OP. I think the answer is "yes".
Is he going to cheat? Even if is bi, by some definition, that doesn't make it any more likely that if he were completely straight. Just changes who he would cheat with.
I think there is a good chance that the OP can find ways to play to his fantasy / kink that might keep him completely happy and would be fine with the OP. That is something they need to talk about to find out if there are things that they would both enjoy.
Yes, he could be mostly gay and planning to cheat or already cheating. He could be cheating with other women. He could be a serial rapist. Unless there is specific reason to be suspicious though, those are just the normal risks that everyone takes in a relationship.
If he's fantasizing about being penetrated by other guys then he's bi or gay there's really no room for interpretation here.