Why is this?

Pretty simple. Marriage is a life-changing event for men. 

Everything has to be adjusted to accommodate a wife. 

Every adjustment has a cost, 

and some of those adjustments require a man to abort his personal dreams and ambitions in exchange for a wife. 

1) A man who doesn't need sex, will probably never marry or date.

2) A man who needs sex, but can get sex without marrying, may never marry.

3) A man who needs sex, and can't get it without marrying, will likely marry.

4) A man who needs sex, and wants a particular female, and can't get her without marrying her, will probably marry her.

A man who won't date without sex is a #2.

A female who gets pregnant out-of-wedlock will be less appealing to most men, including the man that impregnated her.

PS, The leading cause of death for pregnant women is angry sperm-givers that don't want to be fathers.

Sadly, one group of males most likely to marry are (sometimes) fatherless males who look for a woman to support them, 

as their mothers did, establishing a culture of the woman being responsible for the life of the male; a complete social perversity.

Chick flicks are written to make females believe that if she gives a man everything,  pretty much all at once, 

he will fall head-over-heels in love with her. 

That outcome is fairly rare, but to some women, it seems reasonable.

Many females also think they have to compete with other women for a man's attention, is via sexual promiscuity.

As a young man, I spent enough hours in bar rooms to know that easy girls are often viewed as 'semen porta-potties'.

Drop-a-load-and-go.

Sorry to be so blunt and crude, but women claim to want know how men think. .

See also: https://teachustruth.com/1_60_confirmed-bachelors.html

Eric J. Rose