DO. NOT. TOUCH. MY. NIPPLES. EVER.
I hate that.
Other turn offs DURING sex... Things like bad breath, body odor, etc.. will ensure there is NO sex taking place, so I can't take those into account.
People who talk too much. Shut up. You getting fucked. You ain't on Oprah.
Talking about meeting their parents. (had one trick who did this. actually, he babbled on about everything, this parent topic just took the cake. It was like I just gave him an engagement ring or something)
Being a muscle whore. I have had some that liked to rub, lick, bite and TALK to my muscles. It was like I was a seperate part that happened to just be following my chest and biceps around.
Humming. (I don't what damn song it was or why it was stuck in his head, but I did my best to pound it out) You would be amazed at how a tight grip and swift tug on someones balls will get that "song that is stuck in their heads" right out.