Do guys usually ask other guys if they have girlfriends?

It can be, it really depends on the nature of the interaction. It's a perfectly reasonable question to ask a new friend. The fact that he followed the question with "Why not" COULD indicate an ulterior motive for asking, but based on this excerpt alone it's hard to say yes he's interested or no he's not with any certainty. I'd suggest you just get to know him more. Ask more obvious questions without being too super obvious and see how he answers.

Sorry I couldn't be of more help. Good luck!

 

It's pretty normal... unless he's given you some other indication as to why he asked other than just pure curiosity.

 

Yeah, I would say that's normal. If someone is getting to know you, that's a good conversation starter. Unless it was obvious that he had another reason for asking, I wouldn't think too much about it.