It’s not weird to only want to date bisexuals. Some guys are only into bisexual men and that’s ok. Maybe you like the perception that bisexual guys are more “manly”. It’s a bit of a stereotype, but maybe that’s what gets you going about them. Maybe you have a slight interest in the female body so being with someone who likes both interests you.
There are probably a million different reasons as to why a person likes what they like…but no, it’s definitely not weird.