You take out the nature of human reproduction, the length of human pregnancy, the length of human infancy & childhood and the incentive from all that for the human family unit and it is hard to imagine that marriage would have been established, and equally hard to imagine marriage as intended for anything else.
The fact that heterosexual couples may marry with no intention of having children or that they may marry knowing they cannot have children changes nothing about why humans established marriage or what marriage is essentially about in spite of the exceptions we recognize.
No we see from this “gay” activist why, after civil partnership laws were successfully established, and established providing a lot of equality under the law with married couples, they proved insufficient to the activists cause.