When circumstances allow (or dictate, or require) I usually make this point, using gay marriage as an example:" 'God' created the entire universe and everything in it, stars, planets, comets, asteroids, the whole she-bang. And you are so arrogant as to actually believe that the thing this creator god is MOST concerned about is what a single species, on a single planet, in a single solar system, in the ENTIRETY OF THE UNIVERSE
is doing with it's genitals? You really believe that?" in a shocked tone of voice, with a WTF look on my face.
Tends to be either a conversation stopper, or the beginning of a good row.