In my personal experience I see it this way.
Man sees a pretty girl and thinks "wow she is hot!!" then he sees the guy with her and thinks "I want a girl LIKE that."
Woman sees a hot guy and thinks "I want THAT man" then goes after him even if he is not single.
Fact is, men are driven my sexual urges more than women are (in most cases) If a good looking woman wants your man, unless he has very strong morals and willpower, she will get him.