Is my best mate gay?

I have known him for 20 years and he has never had a girlfriend or told me he is attracted to a girl (or boy for that matter). Me and my fiance have tried hooking him up with girls but always he turns us down. The odd thing is he has loads of platonic (but very attractive) female friends, more than us! Now he is best man at my wedding and I feel like I want to know for sure. Should I just ask him outright (like I've got a right to know)? I really don't mind if he is but it annoys me that I don't know. I've asked him casually a number of times and he has always said he is not. Is there really such a thing as an aual man?