What's So Special?

About the number “2”, I mean.  Why should marriage be limited to just 2 people? 

Now that the Supreme Court has broadened the definition of marriage to include gay couples, by what logic would they defend a limit to just 2 people?  Polygamy supporters are overjoyed by the two decisions this week, with good reason.

I fully expect to see not only legal polygamy in the U.S. within my lifetime, but also legalized inter-species marriages.  Why not marry your dog, horse, or goat?  Where in the Constitution can you construe something preventing that?

I'm not the only one wondering these things...