Why is my female cat biting my male cat?

I have a girl and boy unneuter/unspayed they are about 4 months old. I had the girl since she was 8 weeks and I just found the boy in the gutter. About 5hrs after he was here she began to bit his neck and pin him down. She also kept smelling his butt and biting his tail. I am getting here fix next week and I do not need/want kittens. If this is a sexually please tell me to separate them till there fixed or if it is just a dominate thing.