Unfortunately I think it's because the pauline epistles give us that bit against homosex (even though real bible scholar folk question a lot of the terms he uses). Bible readers with a strict anti-homosexual view can say that the conjunction of Paul+Leviticius validates that those particular terms get carried into the "new law" while the other things (restrictions against bunnies, vegetable gardens) get thrown out by Christians as the "old law." (Of course, back to my hobby-horse, Paul also talks about slave-owning...)