Underpopulation?

I've decided to stop naming my posts after whatever song I'm listening to at the moment, mainly because I just don't feel like doing it anymore. So now the titles will actually have something to do with the content of the post.

There was a lecturer on campus today who talked about the constitutionality of gay marriage and so on. Irrespective of my personal views on the matter, there are certain arguments which simply don't work. The lecturer (I can't remember his name) asserted that and increase in gay marriages leads to a decrease of birth rates. This makes sense. But he went on to say that a decreasing birthrate is a bad thing, because the world is underpopulated. My initial reaction to this was to paraphrase Reservoir Dogs - "That is the most insane fucking thing I have ever heard." Then I decided to step back, give him the benefit of the doubt, and question my own preconceptions. I was halfway through an interesting train of thought about farm land will eventually be valuable enough to make it more profitable than developing. But then he kept going, and said that the reason we need more people is that if we don't continue to increase the population, we'll end up spending more of the GDP on retirees. So we need more people to tax. Human beings are the greatest replenishable resource that we have, isn't that right? Yeah, so the population needs to increase until the end of time. Because if it doesn't, we're going to have to change our budget. We should ignore reality because it's inconvenient. I consider myself to be generally conservative, but when we start running up against finite limitations of natural resources, it's time to start making changes, no matter how painful it may be. And it's not like this is an abstract thing years down the road. He himself admitted that birth rates are dropping because it's not economical to have so many children. It sounds to me (to use a Hillsdale cliche) like the market is solving already. The human population has reached the limit of what the earth can sustain, and it has started to plateau. We need to change the way we conduct our business. We no longer have an infinite army of new taxpayers arising every generation. Time to change things.