I've also heard the justifications based on "equality." At the most basic level, if hetero- and homo- are equal, then everything is equal. The scoreboard reads 6,840,507,003* to 0. Everyone on the planet came from hetero relationships. If that isn't enough to differentiate something, then there is no such thing as differentiation. The two things simply are not equal.
So what's the real substance behind the push for gay marriage?
* - Actually it's much more lopsided than that as the scoreboard should record every human ever in the whole of history.