I agree, you've just had bad luck and asking about your date's sexuality won't help.

It's really wrong of these guys to take advantage of you, though. If they aren't interested, they shouldn't toy with you. I'm gay and wouldn't be very happy if a straight guy led me on just because he was curious. People want to be with people who genuinely want them, no matter their orientation. My suggestion is you make sure the next guy you date seems really, passionately into you. If they aren't, even if they are straight or bi, you can do better.