Doesn't actually mean it ends in a polygamy. Nowadays they tag these kinds of manga as "harem" just because MC has multiple potential lovers but in the end only picks one. Ex: Nisekoi, The World God Only Knows, 5toubun, etc.
And imo, all 3 of the ones you picked were really shit endings, lmao. TWGOK literally picked the girl that did the least. Wild. Nisekoi girls were all fairly okay, but they chose the most boring tsundere violent one. Didnt ever finish 5toubun, but knowing who was picked at the end, I don't care to read it anymore.
Personally, I think harem series that dont do an actual harem end should just follow what BokuBen did. Not sure if it was because of the bs first ending that they did or if it was planned all along, but all the girls got their own endings. Essentially everyone wins. We're only a few chapters in atm, so we'll just have to see how this one plays out.