How do I know if my boyfriend is gay?

To be honest, he's not my boyfriend, just a guy I'm seeing casually. He works out at the same gym where I'm a member. He has this incredible body, alpha type, ripped abs and very dark almost metallic skin. I notice that not only are the men in the gym jealous of his physical condition but that he kind of gets off on their attention. He studies other men's bodies just like they do him.
He's tons of fun, hilarious, generous, sexual tiger, and successful in business but I'm worried he could be bisexual. Are his interests in looking at built men simply a way to measure his own achievements or does he find the male figure sexually attractive? I've always heard that lots of gym rats are bi but I can't tell if my guy is or not. How can I figure this out without asking him - I don't think he'd be honest. I don't have a problem with homosexuality but I don't want the risk of STD's as lots of gay men can be pretty promiscuous.
How do I know if my boyfriend is gay?
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