My friends say that I'm too serious because I don't find their jokes funny?

It sounds like you are just growing up at a faster pace then your friends. This all seems a bit too immature for you right now, and it is. Good for you, for not stooping to that level. Don't beat yourself up for not wanting to be around that, your young, 15, you still have time to find a group of friends that you will "fit in" with, this is what high school is all about, finding ourselves. So step out of the circle and venture out to find different people, different cultures, different attitudes on life, types of people. It's okay to want more out of life. As for you friends, how far in life are they going to get by making fun of sex/rape/penis/vagina.............they won't get to far. Hold your head up high, and set standards for yourself, and better yourself. YOu can still be "friendly" towards these people when you see them, but I think you are way too mature for them, and you need a different circle of friends to hang out with. I wish you all the best. Good Luck :)