They do advocate chastity. For one thing, I can't really speak for Catholics, since I am Protestant (woo-hoo!). But one reason why they might oppose homosexual priests is probably the same reason boy scouts would oppose gay scout troops. Not necessarily out of homophobia, but out of safety. Imagine this: you have a male priest who has taken a vow of chastity. Sex is a part of human nature that, like any other part, starts to really get to you once you go without for a long time. Then say that this priest was homosexual. A priest who spends much of his life around other men AND young boys. The chances of a homosexual priest abusing a young boy would be greater than that of a heterosexual priest abusing a young boy.
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This calls for a castration campaign!