It might happen sometimes, not all of the time. That's why people shouldn't assume things of others. You never know what the other person wants, so just be yourself and ask them out. If they say no, that's fine just move on. I asked out guys that happened to be with someone (I didn't know) and no hard feelings, I moved on.
Yes, well, many of them. I've talked to girls that people would consider a solid 10 and they've never even kissed a guy, and very very rarely get approached. They say guys usually think she's taken, out of their league, or stuck up. That's why I always say that the best girls to approach are the most attractive ones cause they tend to be single and tend to be nice too, though most people (especially girls) wouldn't agree with my opinion xD
I definitely think there's truth to it. There was also a study that found that really beautiful girls had less casual sex. I've noticed it on gag too, like whenever there's a question where people talk about how many people theyve slept with, the beautiful girls often have the lowest partner counts.