“...if this is a “fight to preserve marriage”, then they’ll have to show how this will stop straight people from finding each other, marrying, and starting families together”
They don’t have to do anmything of the sort. Marrige has one definition that serves our society. Any attempt to blur that definition is in fact an attack on the institution of marrige itself for the purpose of destroying it. (Which in fact is what they are trying actually to do)
The definition of even heterosexual marriage has been changing over the last few centuries. Instead of being a contract for property between two families, it has evolved into a farce. And most people today can live with that, in some form or another.
Marriage has morphed into another voluntary relationship, that really resembles friendship in many ways. And straight society has countenanced that for quite some time now. They're not going to build the firewall here.