They came from disliking homosexuals, likely from aversion to the thought of men having sex together. You think that's something unique to Christianity? There were laws on homosexual men being castrated in Assyria over a thousand years before Christ was born.
Certainly hatred of homosexuals is hardly unique to "the big three" Abramaic religions (the Jews were also murdering homosexual men well over a thousand years before Christ was born, for what it's worth).
However: Christians are always eager to point out the Judeo-Christian roots of our laws. Whether Canadian, American, or English, our lawmakers have been, for centuries, practicing Christians, and this has been reflected in our laws. Even disregarding laws specifically related o homosexuality, there have still been a significant body of law devoted to imposing the moral views of Christian lawmakers via the power of the state. One could look at laws regarding pornography, contraception, adultery, or various sex acts.
We know that Christians aren't the only ones who have a problem with homosexuality, but we also know that Governor Rick Perry's anger at Texas anti-sodomy laws being struck down wasn't based on ancient Assyrian beliefs. Catholics might not be the only ones who oppose contraception, but we do know where Rick Santorum's fanatical opposition to the Griswold v Connecticut Supreme Court Ruling is founded. Going back further, we know that the gross indecency law under which Alan Turing was persecuted and chemically castrated was not written because of the beliefs of ancient Assyrians.
-k