How do I stay non-perverted?!?!?!?

Ihave like 4 best friends and we go to the park and hang out together sometimes, all they do is talk about perverted things, now my best-best friend has lately been talking and acting more and more perverted (talking about sex, yelling 'penis', and etc..) I dont feel comfortable when they get into this state of mind and clam up, not talk, they think im weird an should loosen up some.(im a freshmen going to be sophmore come fall) I try to curve the convorsation to where is not as dirty, but they always seem to go right back to it. I want to keep my brothers non-perverted as well and actually hit one of my friends when they asked my brother what a penis looked like(my brothers are in 6 and 7 grade) I love my pals, but lately they have changed a bit, I do not want to change friends or lose them over the way they are acting. It may be 'just a phase', but i dont like it. My question to you, viewer, is how do i keep my friends and myself, from speaking about things that make me feel uncomfortable??? Thanks.... :/