Something you don’t understand: Sex isn’t just involved in pregnancy. In most social animals, humans included, sex also plays a major part in social interactions and relationships. (Insert usual caveats about sex-repulsed aces.)
The fact that unlike other social animals, humans can make both connections (well, at least some humans, anyway) puts a great deal of tension between both of those things. That tension is removed when you decouple sex from pregnancy, at least until the people involved choose otherwise, which is why we have contraception and abortion in the first place.
Or maybe you do understand, and you subscribe to old-timey morality where suffering is good and enjoying life is evil, because you’re either a masochist or a hypocrite.