I think what the parent comment is suggesting is pregnancy (and sodomy) overwhelmingly impact the people directly involved with it. It's extremely unlikely (though still possible, technically) that one person sodomizing another will transmit HIV to a third, unrelated party.

I think it's a bit like second hand smoke vs. first hand smoke. I think it's entirely reasonable to object to second hand smoke (refusing vaccinations while insisting on participating in society) in all forms while supporting an individual's right to first hand smoke (pregnancy/sodomy).