Women are naturally more selective than men, and rate partners differently than men. If I remember correctly, the preference of the average rated woman is to go for the top 20% or so of men. This causes those top 20% of men to have many partners, while the rest is left with below average prospects in terms of attractiveness.
Maybe the grass is always greener, but having to lower your standards to some slightly less attractive women for your genes to propagate doesn't sound too bad compared to having to be the one to grow a human in my abdomen.