It's really unimportant if the slut cums or not and neither outcome effects my pleasure in the slightest. If I'm feeling really psychologically sadistic though I like to try to force them to cum whilst being raped in the ass. Then I can tell them they came because they're just a dirty little slutty whore and deserved it all along... Psychological trauma really is the icing on the cake for me when I'm in that that kind of mood