It is all supply and demand. If a women wants to get an affair, she goes to a pub and she can get laid. Not so much for a man. The ugly fat girl can still be not interested in sex (at that moment).
And the ugly fat girl can still get laid if she wants to. Not so much for the ugly fat boy.
Now if you want a relationship, it is almost the other way around.
Many years ago I knew a person who owned a dating agency. There were so many women and so few man he often pretended to be a potential candidate. This was not for a fling, but for a relationship.
Women at that time were much more willing to look for a relationship that way. This was all before the Internet.
It is a bit of a standard that men want to have sex and women want to have a relationship. Yes, there are plenty of exceptions.