>>823>Why are there so many people like who get outright offended by it.
It's because NTR completely subverts relationship dynamics and portrays itself as being relatively "normal." To anyone actually normal -- i.e. people who aren't c*mbrains who get off to anything sexual -- you genuinely have to be a cuckold to enjoy it, which is frankly insulting. Regardless of how much you care about the character, then, seeing it is functionally identical to rape (not to mention, NTR is often accompanied by rape as well...).
>There's much weirder, dirtier stuff out there
There is, but the difference is that guro, scat, BDSM, urethra insertion, etc. are far, far less common. Were it the case, however, that urethra insertion was as common as NTR, you'd likely see just as visceral a response to it as well. So, although people might despise those things as well, they don't actually confront it very often so it's not very much of a concern. By contrast, if you search for just about any series, you're likely to find a handful of NTR doujins.
Part of the outrage is likely also driven by the fact, as others have mentioned, that an increasing number of vanilla artists have been producing NTR lately, which then directly impacts readers who have otherwise been able to avoid NTR.
As someone who very heavily considers themselves to be into vanilla stuff and deeply hates NTR, that's my two cents anyways.