"Bi" these days seems to mean "is sexually attracted to men, knows it, but doesn't want to be identified with gay culture". It's more a statement of personal style than of actual physical desire. Not that there's anything all that wrong with that. I like plain old regular men, too.
I've only known one guy I truly thought of as bisexual. I got together with him after not seeing him for a couple years after high school. I went to where he was living with this hot nurse (an older woman, probably in her mid-20s at the time). She was there, and when I saw him looking at her with love (or was it lust?) in his eyes, I thought my chances of getting it on with him were zero. He was just too into her. But then she went off to her evening shift, and he swiftly got me into bed and fucked my ass for the first time. And that same look of lust was in his eyes for me.
This dude was truly bi. Only one I've ever known.