I really can't think of anything that's ever been said mid-sex-act that's turned me off, or been "worst."
Really, the only thing I can think of at all is the once when my (monogamous LDR boyfriend, reunited for a weekend) and I were cuddling up in bed, and I was trying to get him to share some fantasies or new ideas to try with me. After painful minutes upon minutes of "no, that doesn't really sound very hot," I asked "what about X?" and he said,
"Yeah actually I had thought about that before but I didn't want to bring it up in case you had already done it with someone else."
Maybe this wouldn't bother some people? But I'm usually pretty even-keeled and easy-going, and hard to offend, but I definitely curled up into a noncommunicative ball for a few hours after that.
I think it was the combination of implied sluttiness (in the sex-negative sense) (he never came to terms with my sexual history despite my honesty and then willingness to drop it/not bring up stories, and had said things like "Well I worry that, you're, you know,... a slut" before), and also the fact that after me trying to get a fantasy, any fantasy out of him for months, it turned out that he did
have fantasies and just didn't feel comfortable sharing them with me.