it's less 'do I believe that they would date other people' and more 'do I care ....' Like, realistically, I think most people could probably find their way into a relationship with a variety of other people under the right conditions and circumstances, but I'm likely not going to be interested in that enough to want to read about it! And there are some relationships that I specifically do not want to read about because it goes contrary to the point it feels like the canon was making about their relationship.

Healer is a great example here because I do think it's quite funny that the show wants me to believe that Youngshin has never dated anyone. I am sure she went on dates in college and had a perfectly fine time. But I've got no reason to read Youngshin/any of her colleagues or Youngshin/random college classmate because the show hasn't done any work to make that interesting to me, and I actively would never read Youngshin/Moon-shik because it is important to me that that relationship is not actually romantic but is in fact the specific dynamic between an older cousin who got very parentified and the kid they used to babysit. (I would however read Youngshin/Yeon-hee because their three episodes together were very compelling.)