It is about an Earthman who comes to the planet Vexvelt, shunned by the rest of colonized universe for unknown reasons. He finds it a utopian paradise, but then discovers to his shock and horror that incest is actively encouraged there. When it is shown that the reason that the planet is a paradise is because of this encouragement of incest then questions are raised of the reader as to how we face 'unpleasant' issues.
Sturgeon wrote the story with an afterword that makes it plain that incest is not the real issue here, but rather how we manufacture falsehoods and turn them into perceived 'truth's. How we often take something harmless, then add and build on build on the perceived 'truth' to the point of creating something that is positively harmful.
This story is written to illustrate Sturgeon's credo of 'ask the next question'. By picking a very contentious subject for promotion he prompts a common array of non-answers such as 'because it's wrong'. Sturgeon then pops the question 'Why is it wrong?' and shows us how we tend to flit around an issue instead of facing the reality. The afterword makes the point that each successive answer should prompt a question that punches through it until a proper truth can be reached.