First off the human population is roughly 50% male, 50% female.

A woman wants two types of men in her life, a guy who can show her a good time in her 20's, and a provider and family man later on.

The guy who can show her a good time almost never turns into the family man later on, so they must be two separate guys.

Since about a quarter of the male population is busy showing young women a good time, this only leaves 75% of men left to be "providers".

So at least 25% of women will end up lonely at the end, because there isn't always a guy waiting for them. The human population is 50% male, 50% female.

This is the part no one's realizing, this system isn't sustainable at all. I can assure you we'll see a lot of unmarried people in the future.