The LGBTQ movements (short for Lesbian Gay Bisexual Transsexual Queer) and their support have grown strongly in the last decades. For many people, this development might always have felt natural, but there is still a big group of people which keeps insisting that it's not and that these people are going against the natural order. Assuming it is humanity's job to behave naturally (what is certainly arguable), I'll now explain why that is not the case and how it comes that LGBTQ has been suppressed for so long.
Let's begin with the basics: The main job of sex in the evolution has always been reproduction. But that doesn't mean that's its only job. Sex is also a kind of fun we can have together, that connects us. Sex is often something we do out of love and what intensifies that love. It is like an emotional glue between people. And since the functionality of love and emotional connection goes beyond reproduction and doesn't require opposite genitals, of course it can also happen between people of the same sex. Which means, of course also sex can and should happen between people of the same sex.
In the next part, we need to find out, how it comes that some individuals are not attracted to the other sex:
Let's face it: Sex drive can be a burden. It takes a lot of time, it sometimes switches off the rational mind, and if not fulfilled, it can make people very unhappy in various ways. No wonder there were cases in evolution where those had an advantage who didn't have that problem because their sex drive didn't exist. They might rarely reproduce, but have better access to self-transcendence or be better at things that don't make other people at all think better of them.
Now we have discussed two aspects of homosexuality: The attraction to the same sex and the non-attraction to the opposite sex. Now we just still have to understand how this can happen at once:
Most of us know the experience of being attracted to some types of people way more than to others. Since we live in a society with dominant gender roles, many of us will associate the personality traits they are attracted to with a gender. Homosexuals are people who are mainly attracted to traits that are associated with their own gender. That doesn't mean that their own gender identity would be that of the opposite sex. Attraction doesn't work by opposites only, but also by similarities. If someone needs a similarity in a partner that is associated with their gender role, that person is gonna be likely to call themselves homosexual.
The question is now: Shouldn't it be a goal that everybody can reproduce with a person they enjoy it with? I think yes, so I think a big variety of types in each sex should be supported to ensure that everyone can easily find a partner of the opposite sex. Of course this should not go at cost of people's identity, but I actually think pressing people less into mainstream gender roles should be enough anyway.
After only talking about sexual orientation so far, we'll talk about gender identity now. It's understable to have no idea how transgender should have an evolutionary advantage.
I think that certain abstract personality traits are genetic and immovable (For more theories about personality types look at my Typology Blog). Those traits are in my opinion not inherently linked to wanting to have a different body. Much more those traits have been socially divided by gender roles into masculine, feminine, androgynous and genderless traits. Transpeople are people who have been born with personality traits that contradict the gender role that their body has been pushed into, and much better match the gender role of the opposite gender. The realization, that people with a different body or appearance are treated in a way that makes more sense for their own personality, leads to a desire to dress differently, to have a different body, or even a feeling to be in the wrong body.
This leads me to the conclusion that transgender is actually a result of the gender roles we have today. If people of different gender roles wouldn't be treated inherently differently, but dependent on their actual personality, body transitions wouldn't need to happen anymore. Of course I don't know if I overlook a reason for body transitions, but at least it's what I guess from listening to transpeople so far.
1. Bi-, pan- and asexuality are completely natural and pretty likely won't end.
2. Homo- and transsexuality are mainly constructs of our gender roles, and will probably get less the more we treat people by their actual personality instead of their gender or sex.
3. Until then, just accept that people are in the LGBTQ spectrum, and when transpeople want to be addressed with certain pronouns, just use those pronouns.