I actually rather enjoyed GXP for what it was. It also was at least honest in that having harem "who will he choose zomg?" drama is pointless in a polygamist society like Jurai. Seina did what Tenchi should have done 20 years ago.
I agree that it was at least a unique idea among typical harem anime to have Seina just marry all of them, not just because they all liked him but for various political reasons, as well. In general, one of my problems with harem anime is that the guy usually has a nice bonding/romantic moment with each girl as he meets them but once the group is formed it devolves into a lot of fighting over the guy and romantic misunderstandings. Not always, but I remember that happening a lot in the series I watched 10-15 years ago.
I also felt uncomfortable with how all of Seina's new wives - plus those other women who showed up and wanted to marry him (I had forgotten who they even were by that point) - all aggressively jumped him at the end.