Well...
Ok, Romans (and almost all the other books with place names in the New Testament) were written not by Jesus, but by Paul. Leviticus has the other passage that condemns homosexuality, but it's got a lot of other things (it's mainly a big book of restrictions...
As mentioned earlier, same-sex couples don't make babies, which is what is a big part of the prohibition in the first place. Sex is for babies (remember, they got after Onan too, who 'spilled his seed upon the ground'...
A big portion of Catholicism is about the 'be fruitful and multiply', which, unless you have 'Adam and Even' (which is the name of a local sex toys shop...ain't irony ironic?...
which is also why they hate abortion (like anyone loves it...but that's a whole 'nuther ball of wax), etc.