That procreation argument is silly. Human beings order themselves into societies, and not as some sort of procreation machine. But instead to encourage kindness, cooperation, happiness. We have laws to keep from harming one another, and roofs over our heads to keep us warmer and dryer than we 'naturally' would have been. This is what it means to be human, too.
Oh I get it - argue whatever it takes to make male-female marriage sacred and anything else wrong. Ok. I see.
I would say that marriage and sex get their value and purpose from God. That he created man and woman to come together in marriage and have sex. That he made sex pleasurable, and good, and that one of the benefits of sex is often that children are produced.
I think we must either get our understanding of sex and marriage from a source outside ourselves (the transcendent God) or we just make things up to suit our individual drives - and the second option opens us up as individuals, and as a society, to a lot of harm.
I understand this is not a popular viewpoint, especially on HN, but I think it's valuable to offer some pushback in a civil way every now and then.