In the future there could be a true Kwisatz Haderach (a sufficiently strong human mind) that could reach into both future and past and influence them. Since it wants to safeguard its own creation, it is logical to assume that it would reward those that brought about its creation while punishing those that hindered it. Therefore, having sex and procreating is desirable because it increases the chance of the Kwisatz Haderachs appearance, leading to being rewarded.