Sperm Is A Mutagen
It's quite simple; sperm is a mutagen. It can cause mutations in the cells it comes in contact with. Certainly, it's VERY effective in causing DNA changes in a woman's ovum.
If you don't have sex very often, then your own sperm can cause prostate cancer, because your own sperm accumulates in the prostate. If you're a woman, you can get uterine and cervical cancers that are related to your exposure to a man's sperm after intercourse. If you're a woman who has many sex partners, your risk of cancers is greater than if you have only one partner, because you're exposed to many different types of sperm.
If you are a man who has had a vasectomy, your risk of testicular cancer is increased, because the sperm is blocked in the testes and doesn't make it to the prostate.
If you're a gay man and are on the receiving end of anal sex, you're getting your OWN sperm, AND from your partner; hence the increased risk.