What exactly is the reason behind it?
As a hetero-blam!- man, the main reasons I would get married to my future wife is to get emotional and spiritual consent for myself to have kids ( I'm Christian), be able to as the Flood says "stick it in her poophole", and feel bonded to my wife...
Now 2/3 of those things will serve good for humanity because I'm evolving it as a population....
As a homo-blam!-, from what I have heard, I would pretty much get married for only 1 of those 3 things I listed before (feel bonded to my partner), and it is the one that doesn't benefit evolution at all!
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Wouldn't worry about the lack of kids either, -blam!- couples can have children through in-vitro and surrogate mothers. Or adoption.