Is my boyfriend lying about his sexuality?

We are both male and in our mid 30's. My boyfriend acts very straight. I have been with him 4 months and I'm starting to suspect that he's not gay. He likes alternative music, rock-climbing, running, cycling and works as a geologist. He has a very deep voice and doesn't really have anything camp about his personality. He doesn't like camp music or tv. If you met him you would assume he is straight. He is very passionate in bed and very affectionate in and out of bed. In fact he can't keep his hands off me and loves to kiss lots. It's just all the other stuff that makes me wonder about him. Do you think he's maybe bisexual and lying about being gay?