Sex is not a Natural Act?

For context I am a questioning gray-Asexual and Aromantic.

I just finished "Ace" by Angela Chen and I loved it. Every chapter spoke to me deeply and I found myself agreeing with every one of them.

Except for one.

In Chapter 9: Playing With Others, Chen delves into the idea that "Sex is not a natural act."

Is it not?

Of course many aspects of sex are socially constructed, and what is even considered sex is different for every person, but how is the act of sex itself not natural?

Across hundreds of thousands of years and across so many species, sex has been the way to reproduce. To be honest, sometimes even I don't get it. I frequently wonder how sex just happens. How you can put two organisms in an enclosure and they'll just know to put one organ in another and have intercourse. I don't get it. But I do know that it happens and has happened many many many times. All across the world, regardless of culture or custom, people fuck.

So many "simpler" organisms sexually reproduce too, so its not like they know sex will lead to offspring and thats why they do it.

Of course sex isn't as integral as breathing or eating. Many people and organisms go their whole lives without having sex and they turn out fine. But others do end up having sex and continuing the species as a result.

I'm confused at the idea that sex is not natural. I think it is. That doesnt mean that everyone has to do it, but some people do.

I wanted to read "Sex Is Not a Natural Act and Other Essays" by Leonore Tiefer but I couldn't get my hands on a copy. So instead I turn to you, Reddit.

Please change my mind. This topic interests me greatly as a zoologist and I'd love to understand why people think this way.

Note: When I argue that sex is a natural act, I do not do so in order to argue that sex is "good" in an appeal to nature. I don't care whether its good or not, im strictly interested in whether or not it comes naturally.

EDIT: I believe a new question to investigate is: What does the word natural mean anyway, and should it matter?