>It's extremely unlikely (though still possible, technically) that one person sodomizing another will transmit HIV to a third, unrelated party.
Consider the prevalence of casual hookups, and how long the incubation period is, I say the chance is pretty high. If anything, the aids epidemic is evidence that it can spread despite requiring intimate contact. That said, for the purposes of this argument I'd be okay with limiting the argument to "ban sodomy for non-married couples" to rule out the "third, unrelated party" element.