They dont ask. Most dont know or even wonder until I say something. Then afterwards, its an awkward exchange of "oh youre gay...i didnt know that. I mean, its cool with me...I just didnt know you were gay...but its cool." and usually my facial expression is similiar to this as they express their shock and stumbled over their words:
Most of my straight guy friends are asians (more specifically korean) who are very metrosexual and there are some who are internationals so encountering a real life gay is a new experience for them