Individual happiness. I believe many have natural homosexual tendencies/inclinations and I believe many of them find happiness in that lifestyle.
Personally I think two people trying to lead good honest lives who happen to be gay and who find happiness in marrying their partner is good for society.
Not sure that polygamy = procreation only. From what I understand Joseph Smith didn't have any (or many) children with his other wives, so there might have been other reasons.. Things that I don't understand.
My take is things change (even within firm Church practices or doctorine), and changes isn't always a bad thing.