This topic from Long and Short Reviews gave me pause. Usually when I stop reading a book it's because it simply isn't working for me, not because of a particular topic. However, I then remembered a few I had given up in the past and thought about what had put me off.
First off - rape. I accept rape can be a plot point and that's fine, but I was recently reading a historical in which the heroine had, in the past, been on the verge of being forced to marry her rapist and had been rescued by her now-husband at the last second. In the space of the first three chapters we had at least four instances of either her rapist approaching her at the royal court or her parents threatening to "expose" her if she didn't do what they said, and after the fourth of these I just thought "You know what, I'm done." I was there for the machinations at the royal court, not for all this.
The other main issue I had, having read a lot of short romance stories, was jealousy fiction which uses sex as so-called "punishment", i.e. one character deliberately makes the other jealous so the other will be all "Right, you're totally going to get it tonight!" Maybe it's because I find those sorts of headgames irritating in real life. In any event, I don't want to read about them - ever.
So what makes you stop reading?