As a blogger over at outsider gamer site Border House notes, marriage in Frontierville comes after a series of letter-delivered quests where the future love of your life demands certain chores be performed in preparation for their arrival. Only after these prerequisites are met do you get to meet and customize your spouse, setting their appearance and gender to your liking.
At first glance the option for same sex marriage seems like a big step forward in open-mindedness and acceptance of alternative lifestyles in the world of social games. Then you realize that the wall post that accompanies your Frontierville marriages still mentions a bride and groom alongside a smiling, heteronormative picture. It's quite possible Zynga just didn't think to go to the trouble of forcing spouses to be different genders, and so this feature is a nod to laziness rather than inclusion.
Either way, that sound you hear is probably an army of culture warriors revving up their outrage engines over the idea of Zynga exposing children to such "controversial" content.