best before end:

most children, as they begin to exercise their curiosity about this thing called sex that grownups make such a big fuss about, develop a very clear idea how it is: mummies and daddies do this sexing thing to make a baby or two, then, having successfully done that, they stop. that's it. no more sexing. the idea that mummies and daddies might do sexing for any other reason (than making babies) is outside the frame (ask any teenager). and, of course, as is usually the case with the infantile take on the world - it only takes a moment's consideration to realise that their version is the better one.
clearly, the vast majority of our problems as so-called grownups - physical, mental, spiritual, psychological, political, social - would evaporate if our collective libido were to evaporate after the birth of our first child (or upon our failure to propagate by a certain time determined by the biological clock - round about the mid-twenties, say). then, rather than having to do all that familiar repression and/or sublimation stuff, that hitching of cheesy-smiling happy families denial to the post of teeth-gritting real families compromise and necessity that's all too familiar to everyone (everyone) who's a parent (ie in the real world, as opposed to the celebrity makeover world of limitless resources and limited half-lives), our energies would be properly (and contentedly) directed where they were most effective and most needed - in the highly energetic business of child-rearing and getting on with our lives.
it's slightly strange that such an automatic post-partum neutering didn't evolve as a trait amongst the primates, since it would have led to far more social cohesion in the troop > tribe > family, and proportionately less psychotic behaviour amongst the males, especially. other species have managed for far longer than humans with drones rather than males, queens rather than females, or, where sperm-carriers are still considered necessary, instant despatch (mantises, spiders) on delivery. only we humans persevere into dotage with this delusive drive that's doomed, always, to end in tears.
it would not only be a huge spur to getting on with it and getting it right - the knowledge that your bits were going to blow away when you passed your twenty-fifth birthday - it would be a merciful release from a steaming bucketload of cultural bullshit connected with mid-life-and-all-points-south crises (in every sex) and dangerously inappropriate liaisons of the humbert e humbert kind, with the net result of making the whole world a damn sight safer, happier, and friendlier place.