It's more like not reading a book because it has a flashy cover.
Sex sells but I won't buy a book about physics that has a woman in a bikini on the cover. It's about the signals you send as an author.
"Don't judge a book by its cover" is a lie-to-children; there's some truth to it, but if I were making a list of sayings we tell to children and how truthful it is, this one would actually be rather far down my list. Judging things by their cover is generally effective. The horde of exceptions stampeding into your mind and encouraging you to hit "Reply" and start to list them are exactly that: Exceptions.
There are, admittedly, rather a lot of them. I look over at my bookshelf and I could show you some myself. Nevertheless, it really is a better heuristic than a lot of people would care to admit.
Part of this is because covers have a lot of intentionality in them. People deliberately imbue them with information. I don't ignore them. Mind you, I might not come to the intended conclusions... to take one clear example, the intentional message being sent by romance book covers and the one I receive are fairly different. Nevertheless, in the end, both I and the publisher are generally satisfied by the transaction, even if they might prefer I buy it anyhow, I'm not in the target audience and they know that.
They made no assumptions about the content. They simply expressed a distaste for the cover.
To continue the analogy, there are plenty of physics textbook manufacturers, and not all of them decide to use sex to sell their textbooks. Choosing to not engage with a blatant marketing tactic and picking another textbook is a perfectly valid action for someone of any age.
