Women become more interested in a man when there are other women around him, it's really simple. They trust the judgement of their own kind, thus if a man is popular with women, there's something very valuable of him, be that his character, his money, his sex skills, or simply looks. The opposite is also true, if man is a virgin in his 30s, there's probably something very wrong with him, so it's suspicious.
It's really that simple.