My guess:
For various reasons, people want things other than children. High sex drive is an adaptation that would encourage people to have children even if circumstances are less than ideal; it's not very good for you to be 14 years old, unmarried, and pregnant, even if it ends up being good for your DNA. (Humans seem to have a lot of adaptations that encourage "unplanned pregnancies"; once we had brains capable of choosing to delay reproduction, maybe we ended up with various adaptations to make sure we reproduced in spite of our forebrains?)
It is a cultural universal that people are discouraged from having sex as often and with as many people as they want to. Every culture I've ever heard of imposes many restrictions on sex. I've never heard of a culture that shames people for being too stingy with sex.
If we assume that culture is adaptive, this means that the human sex drive is too strong for humans in society. Why is this? As sex drive is a phenotypic feature with extraordinarily strong selective pressure, why haven't we evolved to have the proper sex drive?
One reason could be that reduced sex drive is selected for at the level of the group, while higher sex drive is selected for at the level of the individual.