We don't, as a matter of routine, nor was there a time when it was. Even YOU conceded that it was something that sometimes occurred under unusual circumstances -- bitterly cold winters, overcrowded communities, etc -- but is not and has never been the norm.
Bitterly cold winters and cramped, isolated settlement are
the norm rather than the deviation for the classic western lifestyle in the past thousand years. That's where we all came from. Having sex in the same room (or the same bed!) where your kids, parents and grandparents slept was indeed quite standard, and not regarded as anything else, either.
The idea that Gideon would face an even more extreme situation only makes it more natural for sex to occur in conditions of limited privacy. General logic would establish the sex and the procreation; story logic would then establish the lack of Malthusian mass death that would normally inevitably result from the diseases of closely packed conditions. And ultimately the civilization would start hitting the much wider and less well understood limits relating to food production and general sustainability - perhaps indeed rather humorously leading to space as such being the limiting factor.
I guess my point is that I praise the consistency between general logic and story logic: take ordinary humans and make them immune to the usual epidemics, and this is what you do get. And governments, religions and technology will be impotent in the face of the phenomenon, and irrelevant.
Probably the introduction of a potent disease was but the first, minor but painful deviation from the local ethical norms; had this not worked, the next step would have been farther away from the path, until eventually the government would
have been lobbing nukes in hopes of killing off a sizeable chunk of the population.