I'd like to coin my own cookie-cutter response for whenever people say "God wants this" or "you must do that" or "there's no doubt in my mind that God wouldn't...". Here it is:
"Do you have any objective evidence to back up this absolute claim? (Please don't appeal to your holy book, because the other holy books also make the same claims)"
Feel free to c/p it and use it whenever necessary. Just be sure to add a trademark at the end.