It's happened to me more than once, not because I act like I'm gay or I give off any gay vibes. Their justification for asking is simply because I've never had a serious girlfriend before. A couple people I've known for literally a few minutes ask me if I'm gay like they're fucking asking me what I just had for lunch. It pisses me off, because it's so damn personal and they think it's their business to know. I'm not ashamed of it or anything like that, but still it's not something I'm exactly comfortable with everyone knowing yet.