The model wife trope... her family has probably been hammering that into her since her childhood. They might even have a fiance lined up for her already...
She might have taken him in due to that. I mean, if she got pregnant by a random high schooler, I doubt her family would see that in a good light. And maybe they would disown her and she could escape them. Maybe that's why she took him in. I mean, her eyes looked pretty dead at that time and only brightened up when she heard his answer about how her house looked like. If don't have anything, you can easily leave and go somewhere else. So she always wanted to leave.
Would be interesting to see, if her house has furniture now, after knowing Taesoo for many years. But maybe not, because her situation hasn't changed.
Still, even if she took him in to use him in some way, I don't think it's particularly wrong. Sure, it would make her not be a saint, but who is one anyway? There aren't many that would do something that is a lot of work without expecting anything in return. Even doing good, because you feel good about yourself when you do good is selfish in a sense, but you wouldn't say it's bad.
If she hadn't taken him in, who knows what would have happened to him. He might have gone back to his family. He might have been stubborn and tried living on the streets, unprepared, in the middle of winter, and died.
Still, I still feel for Nayoon (the childhood friend he sees as a little sister of sorts). It hurts that she most likely won't win.