I was adopted by lesbian mothers at a time when adoption by gay or lesbian couples was not recognized by the state and very difficult to do through most adoption agencies. As such, only one of my mothers could legally adopt me at birth. It wasn't until I turned 10 that I was able to be adopted by my other mother. Both were teachers, solidly middle class, one with a son from a previous marriage to an abusive husband. I was raised well with an emphasis on pursuing my education. I never wanted for food or clothes or shelter; hell, we were first on the block to get a computer. I grew up well.
Now let's imagine that my adoptive mother had died when I was 8 or so. My other mother would have no legal claim to me despite having served as a mother to me since birth. She could have filed paperwork to claim me as an adoptee, but that stuff takes time, and I would have wound up in the foster care system in the interim. How would that have benefitted me? It wouldn't have. But because my parents were unable to marry, that was a very real possibility that I faced as a child. If something had happened to my adoptive mother, I would have been taken from my family.
So please, explain to me how gay marriage hurts families.