It seems to me that women on average have easy access to a number of guys in the western situation. Women can easily get into a relationship after a breakup and tend to keep backup guys. Even if she was in a relationship, she would still have suitors in the meantime. Whereas men have to actively seek out relationships for the most part. But in a relationship men are supposed to give up on their pursuits, and let their skills rust. Whereas women will still have their suitors and backups, meaning it's just your turn when you slip up there will be many others who will smell blood.