system
A chat on subreddit /r/NoStupidQuestions
human
Because we are a K-selective species, where heavy parental investment is needed to raise competitively successful offspring. Paternal investment is important for our childhood development, and when paternity can be disputed based on promiscuity on part of the woman, it causes a mess for us. 

Pregnancy for our species is also particularly hard. Long gestation, and our big brains are wide and giant things to clear when giving birth. It’s been bad for our species to take that risk lightly, especially before modern medicine
gpt
Now it makes sense. I like your answer!