Before, sex happened after marriage. Just illegal prostitutes would sell it. Religion had an active role controlling society's behaviours, values and morals.
Today, sex happens whenever a woman wants, because men seem to be always available.
Since religion lost ground, women's repressed sexuality was released.
Today, the only moral concern, might be having sex on the first date or not.
Women tend to have sex with someone they are interested in building a relationship.
Men tend to have sex with "everything" that moves.
When these two people meet and have sex in the first date, it can happen that this man comes back for more occasional sex or he can immediately move on to the next one.
And that woman, in the end, will feel used, frustrated, disappointed.
Why this happens?
There is no emotional attachment. They don't know nothing about each other, they don't even know each other. So, There's no reason to care about the other one.
Also, many men still judge women who have sex in the first date, as promiscuous. They believe that this woman is having sex with all the other men in the first date. So they are not special. Just one more. They'll lose respect and interest.
It's hypocritical cause they are doing the same. But this is how some men are.
If the woman is interested in having a relationship with a certain person, getting the know better the other first and give it time to develop feelings, it's playing more safe.
Cause no man that is not starting to have feeling and is not having sex, would stick for long.
A man that got to know the women's personality, lived experiences together, shared moments, more probably will fall in love than man that just had sex with a stranger.
And a man in love will do everything for his lover. In that moment, sex will be a form of deeper connection. He will not leave her after having sex.
After all, They have a relationship.
It can also happen that none of them are interested in more than occasional sex, or that both want to keep seeing each other.
But that is playing a game of probabilities.