He may not have commented the way you wanted, but he has come out against it many times. He has always focused on the legal aspects of the issue, that is just his mode. Because someone doesn't put something the way you want or give it the specific attention you want, doesn't mean they disagree.
I'll give you that point. Rush's father was a judge, and his brother is a lawyer. The Limbaugh family is well known in Missouri's legal circles for a long time. He learned from them.
My saying gay marriage is fake is also a legal argument that that can be clearly understood.