I'm not sure I understand what you mean. If I already had strong reason to think that the whole of Catholicism was true, then I couldn't just say "well, but I don't want same-sex romance prohibited, so I'll decide not to believe in Catholicism." That would be fallacious reasoning. But if I start off fairly certain that there's nothing wrong with same-sex romance but am also looking for some sort of theistic tradition that makes sense given what I already know, then Catholicism's views on sexuality would seem to count against it.

Perhaps you could compare this issue to the "it all adds up to normality" sentiment. Even if I decide I have to abandon my old theory of gravity, my new theory better be one that has pencils falling down and not up when I drop them. Likewise, even if I have to abandon my general thoughts on theism, I had better not pick a religious tradition that conflicts with strongly held moral sentiments of which I am still reasonably confident. What's the fallacy there?