It's partly primeval instinct.
Under the reproduction criteria, which still informs our sexual urges, no matter how 'civilised' we think we are as as cultured individuals - we are programmed to look for a partner who is going to help to produce offspring and further our species.
That means men (and this is entirely general because we are talking in an athropoligical context) are attracted to young women, often with wide hips and large breasts because they suggest good child-bearing qualifications. In their turn, women are attreacted to muscular men who will not only provide them with peltny of opportunities for child-birth, but will also provide protection for them, and said offspring.
With our veneer of civilsation applied on the top, men have a subconcious perception that a young woman will be sexually inventive and willing to absorb techniques and ideas, as well as being in thrall to a man who can provide regular sexual activity and satisfaction - which of course, is all men!
Men are not good at reality - they assume that any attraction they feel for an attractive young woman is returned with interest, regardless of how old / out of shape / boring / ugly / lecherous the man may be.
I often find myself apologising for my gender - this is just one more example!