Welcome to the boards, Youngsun.
You know, I think there are always SO many things we can't know when it comes to whether or not someone we want to go out with will want to go out with us. For sure, whether or not they're attracted to people of our gender is one factor, but there are so many more -- are they even interested in dating anyone right now, period, do they feel chemistry with us, do they share some of our interests, do we communicate in ways where we can connect, the works -- that I think not asking someone out just because of one unknown factor often doesn't make sense.
That said, when asking someone out or otherwise expressing interest is also coming out to them (and potentially others), I think that's a bit of a different question and worth at least some basic considerations. The biggest one for me is if you think this is someone safe for you to be out to. Do you feel like letting this guy know you're not straight is safe for you? Do you know or does he seem like someone who, if he's not queer, would still be kind and accepting?