This isn't the same as spoiling your plot. No, really, it isn't. I can see why people wouldn't want to warn for character death per se, for example—that's very specific and plotty and death is a fact of life. But you should warn for violence, especially sexual and domestic violence and torture, because guess what? A lot of people have first-hand experience with those things, or related experience, or just the right wiring that makes reading such material a triggering experience. If warning people upfront that your plot contains violence spoils it, it can't be much of a plot.
Even if you, personally, have been a victim of these things and aren't bothered by reading them, you should still warn, because you're not everyone, and if it affects someone else differently, that doesn't make you the tougher, better model. It makes you different. So warn. It's not hard. Courtesy costs nothing.
And no, I can't force you to do it, but I do think you're being callous if you understand the problem but just refuse to. If you think asking for warnings is OTT, or whining, or "butthurt," or political correctness gone mad, please defriend me.