Twins Strike!!

That's an excellent point. Fakku has tags for plenty of rare
tropes, and some of them (like kuudere) have inspired
great stories.

Let's not overlook common major tropes that are still missing
from Fakku's tag directory, like "camgirl," "foxgirl," "hypnosis,"
"onsen," "pantyhose," "prostitution," "sexbot," and "vampire."

Yeah, based on the names on the nameplate, he married the younger sister, and the older sister retains her maiden name, but regardless, there was no way he could have married both, since this appears to take place in normal, modern Japan. It's still possible that the older sister has a ring without officially getting his name (since they don't actually show her ring finger), but officially, the most she could be would be his mistress (which she clearly is).

Short answer: Because it tends to be the most formal, visible, and thus prosecutable aspect of some pretty abusive misogyny.

Long answer: because it affects that way tax laws work between married couples, and most governments in modern society don't like it when you play with their money. That and most governments in today's society were created with some form of judeo-christian beliefs, which obviously hates polygamy.

the hierarchy certainly shows in the slice of life parts of these manga or as a fetish (humping your boss), but using that to decide marriage? that would destroy any dreams of vanilla this great chapter has.

@Galder:
Maybe you're right about the "better pussy," but the social hierarchy
"bs" is a big deal in Japan, and sometimes it even raises its head in
hentai.

they said he should choose based on whatever pussy made him feel better. and so he probably did. no need to bring this social hierarchy bs into a porn manga.