Ah, but therein lies the real problem. Your understanding of history is seriously flawed, because marriage is not a religious device that government later got involved in. Marriage has existed in many forms throughout history all over the world, and religion has not always been involved. From a Christian perspective, it was bishop Ignatius of Antioch who decided that the church should get involved in marriages and "make them holy". From the perspective of other cultures, some marriages were religious, some civil, some purely personal. At one point all it took was for two people to declare their marriage intentions to each other, and they were married. The state wasn't involved. The church wasn't involved.
So, even your idea that marriage is religious is another arbitrary imposition of a particular individual bias. My marriage is not religious (I'm atheist). Nor is it about population growth (my wife and I have absolutely no intention of ever spawning children). So, even as a heterosexual male, I abhor the attempts that are constantly being made by the anti-same-sex-marriage crowd to cram all marriages into their own personal ideal of what it should be.
...the religions can have it back and gay marriage won't be necessary outside of those few who want to impose their lifestyle choices into religious institutions...
This is a fascinating quote. First, the religions can't "have back" what was never exclusively theirs. Second, it is not those who want same-sex marriage who are trying to impose anything on anyone, but those who oppose it who want to impose their personal biases on others. And third, if we keep the government's role in marriage, we allow the non-religious to have their marriages in their own manner, while still allowing the religions to create their own set of rules around it. If your church doesn't want to marry a same-sex couple, fine. But out here in the secular world, if a same-sex couple want to marry, your church and its members should have no authority to stop it.