Why would someone ask if your gay or straight?

He probably likes you. I mean, I'd only ask someone that ifI had a crush on them. And I think he's just trying to prove to you that he is straight because he doesn't want you (as a straight guy in his eyes) to be freaked out by him like straight guys have in the past. I'd just either ask him if he is gay or not (being sure to assure him that you won't be all weird with him if he says yes) or come out to him. Do whichever you feel more comfortable with if you want to do one of them :)! Hope I helped!!