Not EVERYONE will be having sex, but uni is one of the most promiscuous places EVER. Like it's socially acceptable to sleep around as much as you want basically, so a lot of people take advantage of this, plus the fact that you're away from your parents for the first time so you suddenly control who comes back to your place.
Also though there will be people who get into relationships, or already have relationships, or just aren't interested in sex.
Be careful who you choose to go home with though. I had a bad experience with a stranger. After that I only got with people from my own uni, who I at least know through somebody else.