OT, ksxxs, but what they (churchgoers) mean is that one is supposed to hate the sin but love the sinner - you could therefore hate rape,eg, but love the individual who was the rapist. I see what they mean, just can't see how you'd separate the two.
Back on topic - the fact that spit comes from both partners is a sort of symbolic joining, mingling, whatever, so that you can;t tell where one ends and the other begins.