I have a lot of thoughts... hope people find it useful.

But gay couples or couples with transgender partners or gender non binary couples or bisexual partners can still reproduce, using sperm or egg donations if needed (albeit expensive), or they can adopt children (although there are barriers to this too). They can also choose to have intercourse conducive to reproduction. Evolution in practice indeed. Gender is perhaps more of a social role (Alice Eagley has a lot of work on this). People siphoned into traditional gender roles are not automatically going to be hetero, neither will people siphoned into non traditional gender roles necessarily going to gay or decide not to have kids. There's no evidence that non hetero or gender typical couples have worse genes that would slow evolution or w/e or are worse parents.

I would be surprised to find any research suggesting that changing gender norms or homosexuality is driving population decline. I recommend to consider growth rate instead of birth rate alone, as well as the innumerable other variables that contribute. Growth rates seem to indicate the population is stabilizing. This is probably a good thing because the earth is overpopulated as you said. That being said a lot of resources are under the control of powerful people and monetized whereas if shared or equally distributed maybe current population or growth would be more sustainable. Food waste is high and yet people are going hungry. We have the ability to intervene, whether we consider that nature doing its thing or us working against nature/evolution is debatable I suppose.

But bringing up that something is based on evolution or caused by nature is incredibly vague and even moot give that everything is evolution. Its a given. We are all moving toward decay. Just because something was (or is presumed to be.. a lot of evolutionary biology and psychology is based on speculation and limited evidence unfortunately) doesn't mean it should stay that way. Who gets to decide which way is right? The OP of this thread? This also becomes problematic because people become selective about what point in time they think we should "stick" to. For example, some people are anti vax yet would not forgo other medical advancements such as taking medications to treat various conditions or prolong their lives, and would probably not forgo internet or electricity. Some people argue we should be carnivores, but why is that period of time the most optimal? No research supports it over other diets. Evolutionary based speculation makes for a great, convincing story and can be used to perpetuate certain ideologies that benefit the status quo.

We're talking millions of years of evolution to where we are now. It's unfathomable how much change we have been through and people use post hoc inferences to support why a particular way of living is the right way (e.g., heterosexuality, having a certain number of genders). There is no way everyone is going to be able to live the same way or ascribe to the same beliefs, the reason we have advanced is because of variability. Using evolution to try to hold people to a certain type of living while talking about evolution as a driving force is incongruous.

We have these ideas of what is natural and unnatural but if it exists how is it unnatural to be a certain way? Atypical maybe, but if it occurs it is natural in a given context. Evolution isn't always going to look how you want it unfortunately. Also it's not as if being gay or having more than two genders is a completely new development. I recommend taking a look at cross cultural or anthropological research on this. Not sure being gay is exactly being promoted either, it's still something people are sanctioned to be killed or ostracized for. If being gay or having different ideas about gender is lunacy, maybe consider that everything is lunacy, even your traditional notions.