I am not doubting your experience but I will offer up: A lot of women are insecure, especially about online dating. An insecure person will often profess to like something in order to be accepted or liked. Women are persons. They may not be into rough stuff but may think that's what guys insist upon these days, kind of like removing all pubic hair. A disturbing number of teenage boys don't realize that girls/women are supposed to like sex--and a disturbing number of girls don't either or think there's something wrong with them if they don't like (whatever) or aren't ready for penetration before they're even kissed or touched. And a disturbing number of teenagers believe that a girl owes sex to a guy if he buys her dinner or takes her to the movies or asks her.
I know you're not teenagers but consider what a woman who grew up with that kind of expectation might feel about guys who are doing online dating ?
The 50%+ number is consistent with studies done with large samples of online daters (reporting their preferences anonymously, not just as part of their profile) and with academic research using more general samples (not just online daters).
It isn't surprising. The brain doesn't have a "sexual arousal" network. It has a general arousal network, and arousal gets interpreted by the context. That's why even an intense fear experience can enhance sexual arousal in the right context.
Obviously, real pain would likely pull most women out of sexual arousal, but most "rough" sex acts don't rise to that level.
In addition,
this University study of college students found that women reported reaching orgasm more quickly with rough sex, women who liked rough sex had sex more often, and there was no correlation between consensual rough sex and unwanted abuse in the relationship.
Note that in the OP study, not only did 31% of women say the engaged in this rough sex and it was "never unwanted", but even the 37% of women who had experience unwanted rough acts were not women who didn't ever enjoy rough sex. They were merely women who said that "at least some" (but not all) of the rough acts they experienced were unwanted. Then the final third of the sample were women who'd never experienced it, but may or may not have wanted to. So, that means it is likely of around 50% of women in that sample who sometimes want some forms of rough sex. At the same time, the 37% "unwanted" shows that some men are either engaging in types of rough acts that women don't like and/or not making sure their partner wants it before they try it.
Porn may not be giving unrealistic notions of women liking "rough sex" in general, so much as misrepresenting what types of rough sex they like and how to go about initiating it.
It would be useful for research to use less broad categories of "rough" sex to identify those things that many vs. few women enjoy.
For example, I wold predict that hair pulling and spanking are far more popular than "gagging" (using force on the back of a women's head during oral sex to the point where they are struggling to breathe and their eye's tear up).