Irrationality game: people are happier when living in traditional social structures, and value being part of their traditions[1]. The public existence of "weird" relationships (homosexuality, polyamory, BDSM, ...) is actively harmful to most people; the open practice of them is a net negative for world utility. Morally good actions include condemnation and censorship of such things.

[1] Or rather what they believe are their traditions; these beliefs may not be particularly well-correlated with reality.