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?)
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?)