Just because something seems perfectly OK to millions or billions of people doesn't make it right.
Slavery, for example, used to be considered perfectly fine, everywhere in the world.
Also, there is no State-recognized polygamy in Utah. The Federal government put its foot down and forbade it, and the Mormons folded.
As for Muslims in the USA, as far as I know, they only have one official wife listed by the State authorities. But of course there is no law forbidding any number of people from living together and sharing whatever they wish.
While there might be 1.8 billion Muslims in the world, they are a tiny minority in GMS. GMS serves primarily the Americas (North and South), Europe, and Australia. Yes, there is a representation for the UAE, and for Israel, but the vast majority of GMS players are either Christian or grew up surrounded by Christian culture, so seeing a Bishop officiate at a wedding is normal to them. Amoria used to have the option of a non-religious wedding, but the revamp did away with that.
Real-world considerations aside, implementing this suggestion would take developer effort. "Marriage" in Maplestory right now is defined as a link between two characters. Turning it into a one-to-many data structure requires changes in many places in the code. Allowing only polygyny but not polyandry (one wife, several husbands) would also break the current status of the game as completely egalitarian: unlike some RPGs where males and females inherently have different stats, in Maple there are no functional differences between them.
I honestly can't see the developers making such a big change to this already-deprecated content (Weddings) for the sake of a few players, especially when such a change is opposed by a larger number of other players, as can be seen in this thread.
As a workaround, you can marry your various girlfriends on different characters.