I don't agree that it's not a personal choice, but it's definitely not a choice within a vacuum, that is we're making a choice, but consumerism is pushing us toward it, because shaving watching, varieties of topiary creates an industry and makes money. If we all lived in the seventies there might be the odd naked foo, but they would be few and very far between, because the "choice" wasn't a thing back then. Not that you couldn't, but it wasn't yet something to make money off and so it was highly unusual for anyone to think about it. Equally in earlier years it was unusual for women to shave anything at all. All of these "choices" are based on industry creations and messages telling us that hair is ugly anywhere except on our heads and must be gotten rid of.
All of that is fine if that's what you want but we all have to understand that our choices are only partly personal because they are often "new" choices that are offered to us by the beauty industry or the so-called health and beauty industry. The one huge issue I do have with the bald trend is that women are often told that it is more hygienic. It isn't. Our pubic hairs are there to protect our vaginas from bacteria and viruses. It's fine to go bald, or Brazilian or Hitler if that's what you want, but the full bush is far healthier.
Oh and as for porn. In the seventies and early eighties porn was full of not just bush, but huge bush at times, luxuriant gardens of all of the pubes. Pubes are making a small porn comeback but that is all about fetish and giving blokes something a bit different to wank over when they get fed up with full views of the vulva and vagina and the reason bald became a thing in porn was probably because it made for better close ups of the vag. No hair getting in the way.
In terms of it being recent, it is. Relatively. It started becoming a really big deal in probably the 90s, possibly late eighties in the US. where it kicked off big style before it kicked off here, as do most things body related. It probably got underway in Brazil though, hence the Brazillian, because of the teeny tiny bikinis.
Oh and as for Marilyn Monroe. As far as I know she never said she had no hair from the neck down, or whatever. She was alleged to have bleached her muff so that the carpet matched the curtains though.