My theory to everything ...
In past, women have a pretty mudane low skilled job if any, left the family home to go straight to the marital one, raise kids, cook, clean, etc.
Men, get a job, leave family home straight to marital one, have kids, work til you drop, wife looks after all things domestic.
Nowadays, women have better more stimulating jobs, leave family home to live on their own/with pals/with partner etc., realise they can be financially independent and enjoy pursuits unheard of by the previous generation.
Men, get a job, enjoy all sorts of boy type pursuits, leave family home to live with pals/alone/partner without ever having to comit to anything.
So, women are more choosy than just looking for anyone to set up home with because they have higher expectations. Men, knowing women can stand on their own two feet, know that they can enjoy the company of the opposite sex without having to commit to anything. Time ticks by .....
I know so many late 30s females who are single but who would love to have met the right chap and be setting up home. The ones who finally do meet someone then find a reluctance on the bloke's part to commit. When they finally commit to marriage, they are reluctant to have kids. Basically everyone has enjoyed the good life and experienced so much more than previous generations so they need more boxes to tick before setting up home and having a family.