How the hell would people reproduce? For the sake of argument, let's say the state does enforce such a thing, likely as a reaction to a sudden, unexpected social or ecological catastrophe— how the hell will people reproduce?
As for why I think a society would never naturally take on a homosexual cultural norm (without a tremendous circumstantial pressure, like an unlikely population crisis):
There has to be societal reasons for the development of homosexual cultural norms (as opposed to simple biological ones like the gay uncle theory and kinship selection). Of course, those social reasons become the norm and then could eventually become part of the legal code.
It wouldn't then be necessary for state enforced anything to be needed. For example, the REASONS that polygamy and polyandry are distinct are obvious. Societies that have polygamy (1 man, multiple wives) have totally different economic and social reasons for developing it than societies that have polyandry (1 woman, multiple husbands).
Then there are societies which are just sexually liberal, but this isn't the same as societies in which polygamy/polyandry are the norm. Like, in liberal societies, homosexuality is allowed more breathing room, and in some cultures, homosexuality can be cultural.
In Greece/Rome, homosexuality takes the form of an older male and a younger male. In Medieval Islam, it's also like that. In China...also like that. Um, interesting.
In Japan, female sexual relations were acceptable for young girls, but upon adulthood, it was thought to to be childish, and a woman should set aside such things to prepare themselves for a heterosexual relationship.
The Sambia people of Papua New Guinea engage in ritual oral sex. Older men feed younger men their semen, in the belief that this is how they "grow" up to become real men. Probably nothing even remotely gay about it— it's just culture.
Ancient Chinese conception of homosexuality (barring extremely liberal time periods like Tang China) was more cosmological. Gay relations were frowned upon, though not exactly taboo, as it was believed men are weakening their vital essence through sex. But homosexuality for women was neutral, since they do not lose vital essence through sex.
Modern Chinese conception of homosexuality is more akin to promiscuity. That is, a gay person is just more promiscuous than a heterosexual person, that their sexual orientation is the result of a tremendous sexual appetite beyond the norm, as opposed to a genuine difference of sexuality.
Lesbianism is also virtually unknown, unrecorded, and unremarked upon in almost every culture. Hell, when the Nazis were sending gay people to concentration camps, they only targeted male homosexuals. Female homosexuals were sent to insane asylums and detention centres instead. This is because, society prior to our modern era didn't really understand lesbianism. When lesbianism was first understood to be alike homosexuality, actual Lesbians denied that they were homosexuals. "I just have sex with other woman, I am not gay!" (interestingly, in the present time, according to anonymous surveys, lots of straight people have sex with people of their own genders...but they do not identify as homosexuals).
Also, to clarify, homosexuality is not the same thing as "actions and behaviours" that can be associated with homosexuality. This is important, especially in Theology, where we increasingly find that most world religions said either nothing about homosexuality, or are okay with it. But not anal sex (which is just as binding on heterosexuals btw. Sodomy =/= Homosexuality). Homosexuality (as we understand it) is also a very modern kind of concept, so generally these days we associate that term with a biological basis. Historically, there are multiple concepts of third genders, transgenders, and homosexuals that may overlap but not directly correlate to modern Americanized categorizations. Jewish Law has dealt with six genders (of various overlapping correspondence to our modern LGBTQ+ concepts) for example.
BUT....HOW I THINK A TOTAL CULTURAL HOMOSEXUAL NORM COULD BE DONE:
Sex segregation. Except for mating seasons, humans of different sexes do not mingle. This would naturally create a homosexual culture.
However, as a self-domesticated species, we do not have mating seasons. Domesticated animal species (like humans) are capable of reproducing almost at any time. So it's biologically difficult to justify why such a social segregation needed to be done. Maybe if we have an powerful AI god as a our government and it likes social engineering.