It's so good there's no harem ending. Of course everybody loves them, I also love them, but what's the point of any character developments, if you are just going to throw all of the female characters as harem members? Not to mention that here we have a reasonable explanation, since any other option would mean Yuuna disappearing. And since Kogarashi chose Yuuna as his favourite, which ones again also is reasonable, since he is free to choose whomever and he has shared the most time and near death situations with her, we just can't have that.
Also, is it really that surprising that the main male character doesn't support polygamy? Kogarashi - character setting: prefers to have only one partner at a time. End of the topic.
I would say "Yuragi-sou no Yuuna -san" is one of the best manga in its genre.