They should allow it for no other reason than sometimes people don't always make their character the same gender as themselves.. if a husband and a wife play and they are both playing females, let them marry, why not? That aside, I do strongly support gay marriage and would want to see the game take a similar equal-rights stance, as much of our world continues to evolve accordingly in this direction.
And still that being said... I also know the game is a business. As a result, they have to consider the PR problems and upset partial player base that would result from making the "wrong" decision either way.. because of this, they may just avoid making any decision at all.
Ultimately though, I think if you don't want to give equal in-game marriage rights to every couple, you shouldn't implement in-game marriage at all. That would be the true "We're not making any decision one way or another" stance, as opposed to implementing one-sided marriage and calling it a "fragile" issue to do otherwise.
