So are many heterosexual unions. The thing is, we humans don't have sex exclusively for procreation, nor do we marry exclusively for it. So all this insistence on procreation as some sort of natural, social, or divine definition of wedlock is nonsense.
You can take that up with mother nature!
I am not insisting on anything other than the definition of marriage: A union between one man and one woman.
There are any number of ways to unite. They aren't marriage.