
Originally Posted by
Kraco
You just said it: The only way for that to realistically work would be for the MC to be a scumbag fuckboy who just wanted to take advantage of her. Just like the dudes she has previous lived with, apparently. No amount of other plot devices would matter in a civilised, developed country, under such circumstances. It doesn't matter what she says, either, but she needed to say, more or less, what she did for her to be a victim kind of character with a decent personality. Your suggestion would make the series nothing but a run-of-the-mill hentai title. I'm sure you'll find ones with that exact story in great numbers out there.
You'd need to alter the setting considerably otherwise. One easy solution would be for those two to know each other previously, have some kind of family connection. If he then knew she comes from a really broken family, he might feel the need to take care of her for who she is, knowing that sending her back would be bad. That actually happens in RL, at least very temporarily, although still repeatedly. Usually the officials will get involved sooner or later.
There are plenty of series where an adult (or almost an adult) starts to look after a previous unknown kid. It's an exceedingly commonplace theme in LN/manga/anime. Sometimes it tries to be realistic with formal adoption/guardianship, sometimes it doesn't and the audience just has to accept it. For what's it worth, I actually like the theme, although I hate it if it ends like the Usagi Drop manga.