You all have given me quite a bit of food for thought, and I want to address some topics that were brought up regarding prior knowledge, intention, and consent.
I agree it's possible neither of us knew what BDSM truly meant, and perhaps both of us were naive, but I think the true question isn't whether our ideas of BDSM agree. The true question is that regardless of the label, did he know something about the sexual acts that we would be doing that I did not know, and did he act on this knowledge knowing that I was unaware? Because we could have had very different ideas of BDSM and still have a good time if both parties were communicating and able to give/withdraw consent in a safe environment.
Someone said that the ex may have had a rape fantasy or at least fantasised about violence, and I agree this is possible. I didn't know he had this fantasy, and I think I had reasonable expectation at the time to believe he wouldn't frighten me seriously or hurt me. If we didn't have reasonable expectations about how our partners act during sex, then we all would have to write out a legal document every single time we had sex. But the fact is that we do generally have expectations, no matter what the sexual act, that our partners won't just do whatever they want, and that we are in a safe environment to say no or withdraw consent. I certainly had such expectations and you cannot argue everything goes off the table for BDSM. Actually, to do whatever you want goes against the rules of BDSM, so what he did wasn't BDSM.
So I had some expectations, namely that I wouldn't be placed in a situation where I'd be frightened enough into being unable to say no. I didn't know his meaning of BDSM, and more importantly, I didn't know what sexual acts I'd be participating in. True, it was naive of me not to ask exactly what we'd be doing, but was it naive to assume he wouldn't seriously harm me? No.
Even if he didn't know what BDSM meant for me, I think it's clear that he knew what he was going to do to me beforehand. This type of sex isn't impromptu hair-pulling--this is pre planned violence. Unless he was extremely stupid, he also should have known that I wouldn't expect serious violence or a rape fantasy from him. I can't read his mind any more than he can read mine. It would be dumb and even cruel to assume your girlfriend just knows telepathically that you want to beat her up and "rape" her.
So he knew he would be violent, he knew that I had expectations of being in a situation in which I could withdraw and give consent regardless of what I thought of BDSM, and he knew that I had no clue what he wanted. Yet, intentionally or not, he chose to be violent and submit me to a rape fantasy that I did not in any way expect or agree to beforehand. If you "rape" your girlfriend but your girlfriend doesn't know you want to "rape" her, then that's actual rape because there wasn't informed consent. I don't know about you, but I don't think you "accidentally" act violently toward someone when you knew how you would act beforehand. He didn't "accidentally" act violently, regardless of our different ideas of BDSM.
Again, maybe it was dumb of me to not ask for clarification, but it would be exceedingly stupid for him to assume I would telepathically assume I was okay with violence and rape fantasies.