First, I'm perplexed by the claim that 'don't like, don't read' doesn't apply here- if only because the scene is clearly a fiction, as well as a fantasy; I sincerely don't understand what distinction you're trying to make here. (The thing probably even starts with the standard legal disclaimer that 'all characters and events depicted here are fictional, similarities to living persons are purely coincidental.') Is there any question that neither Karlach nor her strap-on are factual, or that the player can trivially walk away from the game whenever they choose, if it is not to taste?
But perhaps more to the point, the distinction between fantasy and reality is absolutely key for the question of whether consent is an appropriate framework. To reiterate the obvious, there was no sex. Nobody got pegged by Karlach, because there is no Karlach and there is no protagonist, and hence nobody to provide or withhold consent.
Would it clarify things to say 'speech' instead of 'fantasy', maybe? Your own comparison is to making a joke about 'dropping the soap', or other forms of crass humor- but of course we've never used a consent framework for being the target of jokes, either. Crass or otherwise. You can ask permission before you make jokes about people, and in many circumstances it's considered polite to do so, but you don't have to, nor can you plausibly build a society around the notion that people have the right to control what is said to them, or by who. Except, of course, through exercising their own freedom of association. As you say- don't like, don't read.
(I mean, among other problems, how would you get consent to ask somebody's consent for things?)
We do make exceptions for, like, death threats or harassment- speech is not immune from moral considerations full stop, but mere offense does not rise to the occasion. There are issues of libel and reputation, but of course the speech under consideration here is private (except for a few streamers), with no public/interpersonal element extending beyond the subjective experience itself. There are cases where individuals have a duty within their role to avoid crass humor or offensive expression, such as an on-duty cop or a priest, but there is no such universal obligation that we're born in to. There is only decorum, or the lack of it.
Also, I'll point out that Link, one of the prototypical silent protagonists, is one of the specific examples you gave of a character with no moral restrictions on smutty fanfiction. The franchise has certainly matured a bit since the original games were released, but if this is such a bright and clear line, perhaps you'd be willing to provide the specific year at which (according to you) it became okay to write porn about Link? It must be wrong in 1986, when the original game was released, because the franchise was new and that Link was a functionally blank slate, but what about 1990? 2000? What implications did that really bad cartoon show in 1989 have for the morality of Link/Ganon slashfic?