"Next up, pets and inanimate objects as mates."
Actually, I wager that the next step will be blood kin. I expect that in the next five to ten years siblings or other near kin (aunts and uncles with nieces/nephews, first cousins, etc.) will attempt to "marry". You are very correct in stating that once one widens the definition of marriage, the freaks will pop-up in droves.
Hey if homosexuals want to marry, then I should have a right to have 10 wives if I want. ;)
Double yikes, but you got a point.
Two widowed sisters might marry for the benefits. No sex, just married.