"Can we do nothing?" Engeta asked sadly.
"There's not a lot I can suggest," the ship said, "Remember that it is these people themselves - or at least their distant ancestors - who designed this society, and it's clear that they wanted it this way. They have a rich oral history - again, almost certainly a fabrication - which describes how the Wise and Ancient Ones demanded 'all men to labour and all women to carry the seed'. And it has achieved a high degree of social stability."
"What about field agents? People on the ground?" M'Tozo asked, her normal exuberance burned away by the pathos of the situation.
The ship-slaved drone made a noise which sounded very much like a sigh.
"Infiltration doesn't really work in this kind of society," it said, "There's no hierarchical power structure to leverage, no central point through which changes can be propagated. Oh, there are local councils, one per village, with members - all men, of course - appointed by acclaim, which is easy enough in a place small enough so that everybody knows everybody else. But a truly socialist society like this, without powerful leaders, is really hard to influence."
A gloomy silence fell over the shared accommodation section.
"So, we have a stark choice," the ship's voice said, "We can abandon them to a certain death, sooner or later - grotesque rituals and all. Or, we can rip their society apart, rebuilt it in an image we would find more acceptable, with enforced re-education for all or, perhaps, bring them all into the Culture, whether they want to or not, there to demand that they adhere to our norms and values - which is Colonialism, pure and simple. Which is the most moral choice?"
Engeta looked at her friend, who shook her head sadly.
The General Contact Unit Displacement Activity powered away from the planet Tellus, heading in the direction of the General Service Vehicle Overwhelmingly Underestimated. It carried a dispirited and despondent crew, at least one of which had requested long-term Storage with a revival criterion of 'Contact develops a reliable and moral intervention strategy for planet Tellus'.
The species on that planet remained unContacted, although the GCU had emplaced a myriad of monitoring and sensing devices intended to keep an eye on the development of the civilizational remnants, and which might just be enough to warn the Culture in the event of an impending disaster - although there was nothing to guarantee that a Culture vessel could get then in time to prevent an apocalyptical catastrophe.
It was, as the Displacement Activity privately admitted to the Overwhelmingly Underestimated, the kind of unsatisfactory outcome which really should be a little more prominent in the briefings to Contact personnel and those considering joining - which admittedly tended to be packed with rah-rah, gung-ho success stories: that, in reality, the Culture is neither infallible nor omnipotent, and there are some problems which just cannot be fixed.