As long as the guy knows she is on and the girl is happy with it then there is nothing wrong with it. Adults can make decisions.
If the guy doesn't know then it can be a scary when you look down, especially it is a one night stand.
Personally, as a guy it depends on the girl. I did it with a girl a couple of times, but she did not seem to get any satisfaction from it, thus I disn't really enjoy. If I am in a relationship with a girl and she still finds satisfaction during it then I would have no problems with it.