That's a pretty common feeling, and one that I certainly had as I entered college. What finally changed my mind was four years of gender studies classes.
So let me do my best to provide something of an explanation. It isn't really a problem that there's one tutorial or one website that pokes fun at weight issues or assumes it is serving a primarily male audience. The problem comes when those tropes become universal, or at least fairly wide-spread.
Sexual harassment might be a good place to look for examples. A single sexual comment or a single locker-room butt-tap, in most cases, is not that big a deal. But these small acts don't happen in isolation! They happen in a context that has structurally disadvantaged[1] entire classes of individuals (predominately women, in this case). And no, these acts are not the only thing that created such a structure, but they work to make individuals in those classes feel uncomfortable, threatened, and generally unsafe. That's why you'll often hear such acts called "micro-oppressions." And they don't just make their victims feel bad. They also teach the privileged class that the victims belong to a class that it is OK to denigrate. No, not on their own, not in isolation, but in combination and spread over enough time.
As an upper-middle class, cis white male who dates women, I didn't really buy this argument until I had a very particular experience. I was at my university's gym using a cardio machine, and some very muscular athletes were lifting weights behind me. I had forgotten my earbuds that day, so I could hear what they were saying. Mostly, they talked about women and parties. But eventually, they started talking about how all the "non-athletes" on campus really hurt the aesthetics of the place. "Especially those CS majors with their screen-tans," I recall one saying. This turned into a myriad of jokes that I felt were absolutely directed at me personally. They commented how "tech types" only show up to the gym to do cardio and always wear "ridiculous T-shirts with company logos on them," which, at the time, I was. I tried to ignore it, to just have "tough skin," but as they kept talking it slowly wore me down. I left, took a shower, and headed to the computer lab and had a great conversation about Golang.
Now imagine you didn't have a computer lab to go to. That no matter where you went, you had to hear those comments and those jokes. Every time you went to work, you'd hear it from your boss. Every time you turned on the TV, you'd see it in ads and in sitcoms. On billboards, on the street, in movies, and even in programming language tutorials.
Further, imagine that if you were to ever speak up and talked about what was making you uncomfortable, you'd be told to "grow tougher skin" or "just learn to take a joke". Personally, I can't even imagine what it must be like to live life as one of the classes that America structurally oppresses. But I feel like it must suck. A lot.
So what can we do? Well, we start shifting the culture. We start creating safe spaces. We hold people accountable for their comments and jokes while understanding that they didn't mean any harm. We educate and explain.
But most importantly, and I cannot stress this enough, we need to listen. We need to listen to people when they tell us that we are doing something that bothers them. We need to understand how our actions transform spaces that we love -- like functional programming -- into hostile areas for so many people. Because honestly, if we don't listen, we won't know. The author of LYAH isn't a bad person. Those athletes weren't bad people. No one does this sort of thing intentionally.
Yet we have to take responsibility. We have to learn, we have to get better. And the only way to start doing that is to make everyone's sensitivities our problem.
edit --
Perhaps I'm wrong. I'm open to that possibility. I'm the product of a very liberal gender studies education from a university in Arizona, where there were plenty of examples of undeniable xenophobia and homophobia. After all, if the sum of our experiences doesn't fully constitute us, it certainly plays a big role.
[1] It is very difficult to argue that this isn't the case, considering the wealth disparity amongst men and women, especially in the tech industry. If you've got yourself deeply rooted in some sort of MRA thing, then just think about race instead. Wealth disparity between white and black men is pretty wide.