certainly I've no idea how sexual orientation in human works but my observation is, in 'heterosexual' world, it depends on gender 'appearance' (ie masculine or feminine bodily traits), gender identity (what you identify as personally) and sexual interest which can depend on current sex organs. different people have different weights on them. if not all of them are seeing you as an exploration or kink (I've seen that in men but do not know much about it), my guess what is happening here, is when you ask him if he is bi he thinks that the second aspect (identity) is uncertain and that disinterest them from being a partner. this might feel deceptive to them as well because they probably weren't interested in people identifying men in first place.

respecting your original post I'll not comment on that but you can try to analyze it from that angle.

I think you can try dating explicitly bisexual men to see as well.