I don't understand this. Anal is gross. That's a shit hole. Its not a v@g1n@. That's where the p00p comes out...
I don't think I've seen a single story where that was entirely true. There's always an excuse for why it's clean in universe, usually via magic. Not to mention most stories completely skip out on even mentioning that the characters need to take
any sort of break in the first place...
So without that one argument... Why not? Frankly,
normal sex is gross. Like, vomit inducing gross. Hell, naked people are gross, no sex involved. The only reason people survive as a species is because their brains are wired to ignore the grossness temporarily when they're horny.
And that's even easier with a book, since you don't have to physically sense the grossness. You can just imagine the parts that you like, and not imagine the parts that gross you out. In that sense, it's even better than real life...