Assuming that humans live for ever, we would reach a point were the earth would fill and the last two unmarried people remaining will have no choice but to date each other or not marry at all. amirite?

Or they could choose to remain unmarried and happy.

Where is your head at

If humans lived for ever, I imagine the general interest in being permanently wedded to someone would drop off pretty sharply. If there is no end to life, I can see people embracing the chance to share it with different people more enthusiastically.

So, if your scenario ever arises, they probably won't have to wait too long.

Murder is more likly then procreation