Evolution is not an intelligent process, it's the result of random mutations. The more complex the adaptation the less likely it is to occur. A mechanism for "have sex whenever possible" is much more likely to occur than "have sex except for this long list of exceptions", or even less likely "evaluate this abstract function about genes". Complexity only evolves when there's a real fitness benefit. Evolving a mechanism to specifically avoid sex after menopause would have very little influence on your reproductive success, so there's no reason for it to be selected. The sperm banks example is interesting because that one actually does have a large reproductive fitness impact, but it's also too new and too rare for any sperm donation drive to have evolved. But maybe if the situation continues for long enough humans will evolve to want to donate sperm.