Yeah, I think thats because the reason you were turned on in the first place was because it was kinda gross and/or wrong.... If you remove the element of being able to be aroused, (ie the refractory period right after ejaculation), then all you have left are your normal sensibilities, which tell you: dude, this is kinda wrong. I think that's how arousal works in the first place... and the reason sex is subversive in our culture as well