Think of it like this. Hate sex exist, right. Obviously that means that two people don't like each other to have sex. It's the tension.
But, in this case, it seems that Geumsun and 704 have a mutual understanding and have sex to gain that yang energy (or whatever it it that they get from having relations). So this wouldn't be rape if they both have a mututal understanding that they have to do this. Hence, why 704 might show his unwant at times, but still goes with it. And, thankfully, this doesn't seem like coercion either.
Unless, someone totally doesn't wanna do it and the other party doesn't listen then yeah, that's rape.
Am i the only one who doesn't like Geumsun, like he acts all sad in the last page but, I DIDN'T FORGOT HOW YOU TREATED 704 YOU BITCH, idk i don't like that relation 704 deserves wayyy better..