
Probably a cultural difference but, back in the 80s/90s there were popular shoujo mangas where you live under the same roof as your crush. One of the most famous was Marmalade Boy, where the parents swapped couples and the kids caught feels for each other. It's a well known enough trope that no one bats an eye.
Going further back, people did actually adopt potential wives or sons into the family. Kind of like an arranged marriage with extra legal steps. Basically, as long as you aren't too closely blood related, no one really cares because they don't view it as incestuous.
For the same reason, some Japanese people also view going to a cabaret or using a call girl service not cheating. You don't have to agree with it, but that's just their cultutral history. Of course not everyone thinks this way in Japan either.
Y'all consider not-blood-related siblings incest? My guys.... marriage is literally just signing a piece of paper. And a lot of gay people in Japan get "married" by adopting their partner because there is no civil partnership over there. There is nothing incestuous about this. They aren't blood related!