Marriage through the church is of course up to the church. If they believe their religion forbids homosexuality than its well within their rights to deny wedding a homosexual couple. However the states should allow marriage through the state. The idea of homosexuality being a bad thing stems from religious teachings, and of course the church and the state are supposed to remain separate. Gay marriage in no way affects the other people in a negative way, and therefore should be legal. The purpose of laws is to protect each individual's rights from being infringed upon by someone else. Preventing homosexuality is itself infringing on the individual's rights.
As for it dropping the human population it wouldn't. It may slow the growth rate, extremely slightly, but it isn't going to make it decline. The growth rate of the human population right now is absurd and the percentage of homosexual humans is relatively small. It would actually be a good thing if the human population began to decline, but thats not going to happen for a while especially when you consider places like India and China, which have the highest population, are still having tremendous growth rates. But now that I'm thinking about it, with things like artificial insemination and surrogate mothers, theres really no reason why growth rates would fall. A gay couple can have a child these days.
Which brings us to an interesting topic, homosexual parents. I think this is a tricky issue. How does having only two mothers or two fathers impact a child during its infancy and growth? The surrounding environment plays a large role in the growth of a child and their personality and even their physical development. Different interactions between infant and mother work to release different hormones that impact the infant. So what effects do the lack of these interactions have? I'm not sure if enough research has been done to have a clear answer, but its an interesting question.
Despite those possibilities, I think homosexual parents raising a child can go just fine. There are certainly worse situations for children in that world than having two dads or two moms.