While I'm not a Catholic (Roman or otherwise), I always understood that homosexual intercourse was verboten in Christian doctrine because the Bible was very specific in this matter. To wit:
- Leviticus 18:22 - "Do not lie with a man as one lies with a woman; that is detestable." (ancient Jewish law)
- Romans 1:26-27 - "Because of this, God gave them over to shameful lusts. Even their women exchanged natural relations for unnatural ones. In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed indecent acts with other men, and received in themselves the due penalty for their perversion." (The apostle Paul, referring to past humankind in general)
There are other verses, but the words interpreted there as "homosexual" or some equivalent are sometimes ambiguous translations. The above two are pretty specific, however: neither Old Testament nor New Testament doctrine left much room for misinterpretation. The argument that Paul's position only prohibits "unnatural" homosexuality presumes that there is such a thing as "natural" homosexuality, which is a completely different discussion altogether.
Of course, I'm given to understand that even oral sex (even by married couples) is frowned upon in strict Catholic doctrine. So I really don't see why the above should surprise or concern anyone.