I get this a lot too. As an example, this New Year I threw a little dinner party for my friend who had his birthday and three other people, two of them who were a couple I hadn't personally met before. When went outside for a smoke with my friend, the girls were talking among themselves and one of them asked the other if I'm gay. I heard the conversation the following morning from the other girl since I'm regularly banging her. Ironically, she confessed to me that she had been certain I was gay one morning when she was lying next to me, sweaty and still out of breath.
I've grown used to it, I've heard this **** ever since high school. I used to think it's because I look like a *****, but apparently that's not true. A lot of my female acquaintances have thought me to be gay for years now when they first met me. It could also be because I keep my sex life separate from my studies/work. I've had a few talks about it with them, since I'm curious about stuff that concerns me. Usually they give bull**** answers like "you just give off a gay vibe" or similar, but a few times, especially with my recent FWB, I get interesting discussions. For example, I've heard answers like "You look too good, face and body, to be single and straight", or "Your body language is too sensual for a single straight guy" or something about the way I look at other people, the way I walk or the way I talk to others. I know it might be just my vanity nitpicking the answers I like but I've heard similar answers too often to lay it down on my ego.
Also, as a funny little side note: I know a few gay men and they never thought I was gay. Go figure.
Some of the answers seem to rely on the female ego; It seems some of the girls I've asked about it feel uncomfortable because I'm single and straight, oddly enough. One said she fel insecure and inadequate with me before I ****ed her. I've only ever head a guy think I'm gay once. Don't let this **** get to you, go with it. If you look at least decent with a good body and adequate grooming, I think this means that you're good looking enough for it to be noticed, unless you're a limp wristed sissy type.