I really like this question!
I'm not sure how you meant it, but it's interesting to think about. We really are over populating our world. And sex is how we're doing it. Sex doesn't always lead to pregnancy and birth, but our population isn't growing because we AREN'T having sex.
Then there is the psychological component. The desire for sex is ingrained in all of us. In religions, communities, or cultures where sex is prohibited, there are often issues of sexuality that manifest themselves in unhealthy ways. When human beings are forced to abstain from sex, the need and desire for it doesn't just disappear. Pedophilia, rape, and molestation can infect a population.
Interesting question. I saw a few people responded that not having sex wasn't "the answer", but you never said what answer you were looking for, or really what the context of your question was.
If the issue is overpopulation of the world...we could always take the route of futuristic novels depicting societies with strict rules and dangerous control over the population. You know, make people infertile. Then there could be a revolution and millions of people would die. Then we would have to re-fertilize ourselves and repopulate the world and eventually find ourselves in the exact same situation. So not a great solution.
The point is, you're dealing with the human race. No matter what we try to do to fix things, we tend to only find more creative and abstract ways to just mess everything up.