"I swear, it's the only way to remove the curse. It can't be done in any other way." I swear that it's so stupid that the author had to justify to drawn them doing it in an onsen by using the curse. I swear it sound like a scheme.
I personally don't have a problem with it, and honestly I don't think we need another drama-filled arc where someone's nearly consumed by the curse. We already went through that with Elsa, and a repeat would just feel jarring at this point.
Plus you gotta remember the curse can't be dispelled with normal healing magic. And we've learned here that it's especially true when it's traveling through the skin. Up until now, the only way to deal with it was transferring it from one host to another. Mana transfer itself isn't even a new concept in this world, we've seen characters move magic between people before. So yeah, applying it directly inside the body through intimacy, is actually a refreshingly creative workaround. Not necessarily to fully expel it, but to at least minimize its grip on them. The whole "onsen justification" might seem a little contrived on paper, but when you look at the actual mechanics laid out, it makes sense within the world's logic.
Thanks for the translation!