It is a truth almost universally acknowledged among contemporary people that some groups--teenagers, college students, poor people, Africans--simply can not be expected to have the kind of self-control necessary to refrain from sex. They are simply not capable of it. Everyone knows that. And yet, when we bring our attention down from groups to individuals, suddenly everyone is capable of it, especially if they're individuals with whom we are romantically involved. No woman says of her husband, when she finds out he's had an affair, "Oh, well, I guess it was unrealistic of me to expect him to keep his pants zipped." No young man seeing his girlfriend off on a weekend trip reminds her to take along some condoms in case she meets someone and just can't control herself.
We feel perfectly free to expect self-control from the individuals around us, but we deem it naive to expect self-control from certain groups.
Last time I checked, groups were made up of individuals.