Every human being has a sexual orientation. Men called to the priesthood are not eunuchs; in fact, a eunuch cannot be validly ordained. Have you ever met an asexual human being? I haven't.
What if you were only turned on by bestiality but had never actually given into your urges? Would that be OK? What if you really always wanted to kill someone but had resisted or your biggest urge was to set fire to some great public building but you managed to keep your arsonist tendencies to fantasy only? Would you still be allowed to be a priest then?
Yes. The priesthood is made up of men, so it is, by definition, made up of sinners.
Let me try a different analogy to make my point. Isn't a celibate saying he's a homosexual a bit like a tee-totaller saying he's an alcoholic? Let's say he's never had a drink before in his life - he just somehow knows he's an alcoholic.
Homosexuality is a disordered condition. They don't belong in the priesthood. Not good candidates for maintaining chastity.
But not all disorders are equal. So the fact that the priesthood is made up of sinners is beside the point. The 1961 document makes this plain: "Those affected by the perverse inclination to homosexuality or pederasty should be excluded from religious vows and ordination." So the right question is not whether priests are sinners, but whether homosexual inclination is a disorder severe enough to warrant exclusion from ordination.
-A8