Isn't focusing on religion a bit of a strawman anyway? Opposition to gay marriage also has its roots in human survival and biology. Hetero relationships are necessary to prevent human extinction. Natural selection favors hetero couples to keep the human race going.
If 100% of marital couples were gay, the human race would go extinct in a couple of generations. Hetero marriage contributes to the common good whereas gay marriage is detrimental to the common good.
Just as I've said quite a few times, in order for us to survive right now, we need to drastically reduce the earth's population. If common good and human survival is a factor at this point, reproduction at this rate is going to be the downfall of human race.
Then again, you know full well that the percentage of homosexuals are way less than straight people. And still many of them have children through surrogacy anyway.
Married people are responsible for each other. They give each other a sense of security. They look after each other. You can't limit marriage to just rearing children.
