Why did Religion view sex as bad?

Okay so yes all religions are man made, but why tf did humans make having sex with someone you're not married to as bad as committing a murder and like tbfh maybe even worse ( Muslims specifically act as if Zina is the ultimate crime a person can ever commit, every fucking rule is related to it somehow lol) I really don't understand how can someone be literally so bloody dramatic about such a natural human act lol, it baffles me.

I once read a theory that says it's because they had to select a very natural thing and put rules on it in order to control us, like cmon if eating potato peels was forbidden no one would even care or even yk do it and go to the priest and ask for God's forgiveness , and tbh it does make sense but I don't think it's the strongest argument out there and I'm sure there are many other theories that I'd love to hear.

So please share yours✨