Sustainability means keeping the population at a consistent level. Otherwise, exponential growth means that the planet is eventually covered in humans to a depth of 50ft, and nobody wants that.

There are only two ways to accomplish this: having fewer children, or killing more people. The latter is what has historically dominated, but we've become so much better at keeping people alive in the last couple of centuries or so that it's no longer working. That aside, many people find that solution to be tasteless.

That leaves having fewer children. Contraception is the only way to accomplish this. Yes, technically it is possible to accomplish this by avoiding sex. But socially, this doesn't work, because people simply don't do that. Contraception, however, does work.

It's the only available solution to a serious problem, and that's why it's on the list.