Scat...I met a guy once who told me that he thought shit was "the smell of sex" and I was all OH OKAY BYE NOW I HAVE TO LEAVE THIS CONVERSATION IMMEDIATELY AND NEVER RETURN
Vomit...I have had not one, but THREE separate incidents where guys were so wasted that they threw up while we were fucking around...one of them
on his own face, and that guy was surprised that I didn't want to hook up with him again!
Blood/knifeplay...I was seeing a guy once who was really a super nice guy and very hot, but he was really into playing with knives during sex. Though he understood I wasn't into it and never pushed it, just knowing that he liked it was too much of a headfuck for me and I broke it off with him. I have had bad experiences with cutting in the past, so it's not really something I would ever be comfortable associating with getting off.
Banal conversation during sex...there is a lot of different verbal activity which can happen during sex which I am okay with...maybe you're a moaner, a grunter, you're into dirty talk, even if you want me to call you daddy I will do it as much as I think it's silly and weird...but if I am going down on you or someone's dick is in someone else and
that's the time you think it's okay to ask me if I like my job or what kind of movies I'm into...INSTA-MOOD-KILLER.
Animals and kids...not cool for obvious reasons.
That's probably it as far as definitive deal-breakers....there are other things that will vary from guy to guy....eg: there is a guy I hook up with every now and then who doesn't really like kissing at all which kinda bums me out because kissing is a big thing for me, but he is so good at pushing my buttons in a lot of other ways that it makes up for it...but yah aside from the above (which I think are pretty extreme), I am generally open-minded enough to at least try something new if I trust someone enough.