And this approach is winning, because the other side has nothing except "let's let the people decide." Seems pretty weak in comparison. Few of those who oppose homosexual "marriage" can bring themselves to call it unnatural, perhaps because they're afraid of alienating a constituency, but more likely because they're incapable of promoting that argument. In fact, most legislators are in the camp of one lawyer/legislator who I heard say, "I still don't know what 'natural law' is." Sad but true.
The fact is, most people in our culture who are no longer practicing Christians (or Jews) have nonetheless internalized the principles that define the way of life that has made us so successful. Many of us (for I am one of these) truly extol devotion to marriage and family without understanding the word "sacrament."
The opposition knows that, due to the fading influence of the church, we are now dealing with a "softer" form of tradition. It has been removed from theology, and they argue (and believe) that devotion to marriage and family is just as available to gay couples.
But from a purely cold, calculated public policy perspective, it is nonsense. How many gay couples are going to produce healthy booming families, that pass their family-producing practices down through generations? Almost none. So all that remains is that they are selfish. They want what others have. Well, the government doesn't exist to protect the feelings of those who are different. It exists, in great part, to preserve institutions like the rule of law. "Gay marriage" is only a distraction from this purpose.