“What is, or is there, a non religious argument against same sex marriage? “
1) Marriage is most needed when there are children. Children need the stability of married parents. The parents need the commitment of each other, as raising children is hard. Most Toeva couples do not have children. Hence they do not need to be married.
2) Toeva couples (at least the men from what I read) do not believe in physical exclusivity. Lemashal, you may have heard of Jews who believe in keeping kosher only in the home, but eat anything outside. Same here. Their definition of marriage is not the same as everyone else’s.