We may never get an answer why people do what they do. So trying to figure it out is usually a waste of time. All we can try to figure out is why WE do what we do. So I’d focus on that. Not sure why you’d want friendship with this guy. He sounds like bad news regardless of his sexual orientation. .

As about why he is asking all these weird things and why he contacted you even though you were never friendly, he is likely coming on to you. He is trying to pock around and see if you’d be interested as well. That’s why all the questions, otherwise he’d not care.

As about label of bisexual. Many do not like the label because some ignorant people assume that the person can never settle with one gender and will be cheating etc There’s a lot of negative assumptions about bisexuals coming from both gays and straight. Some people might be apprehensive about disclosing it to others. My daughter is bisexual, she had long term romantic cohabitating relationship with both genders and is now married to a man. She was never secretive about it and of course her husband knows and so do other people but I don’t know if she is screaming about it from the roof tops at work. And why should she? (I don’t know if she does, I just don’t know why it matters that people- besides your partners -must know one is bisexual. Who cares?)

I’d not be asking people why they marry one gender if they are attracted to the other. Being bisexual doesn’t mean you must be with both genders at all times. Perhaps he got weirder out by these questions the same way you were weirder out by his.

Regardless of his labels I’d likely move on and not look back.