Butt stuff

I'm a freshman and almost 2nd year at Unni. I just started really dating and enjoying male company. But everytime I'm with a boy at some point in the evening ( smooshing) they always try to place their wanker in my bum. I don't know why. But it's happened 3 times in the last 2 weeks. It kinda hurts and it's not all that pleasurable. But boys always want to do it. What's the deal too much porno?