I know, I shouldn't really question world-building in series like these too much, but in a world like this wouldn't it be normal for society to tend towards polygamy? If not immediately on a legal level, but at least socially? Among Sera's college group 3/4 have a boyfriend, which makes me assume that it is already the case anyway. But the author is leaving this topic unclear to create artificial conflict. At least that is my take on it.
Its not certainly a bad thing tho? Its probably because:
1. Its not time yet. The story between the 5 established girls so far are still separate outside for some moments where they cross for like 2 line of text. So I think Polygamy is gonna play a bigger role when all their story are mixed in and they have more interactions.
2. Imo, the reason why the author hasn't touch Polygamy is because, out of the 5 established girls, 4 of them all have stigma towards men or how men act in their world. They're all cocky womanizing aholes, thats why they are attracted to the MC cause he's the opposite of that. They're borderline anti harem.
So i kinda agree w you that the author is leaving this topic unclear so they can generate as much conflict as possible. However, imo, it makes sense. And its somewhat necessary. Established all the characters first and then smash them together makes a better impact that just straight harem of the bat. If you want the fire to burn, you gotta have to feed it