It makes sense that in an old-fashioned culture such as the PS setting is set on there is no room for same sex marriages.
I find that pretty easy to understand and I have no problem with it at all. And mind you, it
affects me.

And the reason why other MMORPGs allow it is probably that they are not really RP games.
There is no reason why you can't RP whatever you wish despite the surname of your characters, labels are OOC anyway.
You can also RP an unofficial ceremony and nothing will stop you from doing that.
In the end, that's what marriages IG are... well, that and clicking a button. I don't see the RP part of clicking a button though.
But this has been discussed many times, and most of these threads have been locked or deleted.
So expect this one to be soon.

I've been told by one GM they are only aloud to change a char's name if it is not aloud or if it's marriage or divorce, and well this reason is marriage.
This is not true, and I don't mean that you're lying. But whoever said this did, unless the policies changed in the past weeks.
I've recently seen TWO couples of brothers/sisters IG and both had the same names for the two couples.
Those are new characters and they have been recently renamed, so if GMs aren't going to change names now for you I guess this is more about making sure the mechanics are not overlooked.

But I'm pretty interested in this, since it seems that no longer is possible to create brothers or any other relatives... if what GMs told you is the way it works.