cvc, thanks for pointing this out. I note that it's a British study. So maybe it only applies to Brits!
But seriously, they did not say that sex causes prostate cancer. Rather, the two occur together--men who had a lot of sex were more likely to have prostate cancer. The suggested reason was that men with high sex drives probably had high hormone levels, which might contribute to pc.
Previously, someone else here pointed out a study that linked pc to acne. Which is also linked to hormone levels.