My mistake on that claim, I still roll my eyes at people who say it's totally normal and okay. How many racey pictures of men have been canonicalized like that? There's a leering element to it.
To be fair, very few other racy pictures of women have been canonicalized like that, either; about the only pictures I can think of are paintings, specifically the Mona Lisa, which is hardly racy, and Boticelli’s Birth of Venus, which I suppose you might look at as racy, though I wouldn’t particularly. There’s a couple other non-picture works of art that are canonicalized in that way, but aren’t themselves pictures – Michaelangelo’s David as mentioned in a sibling comment is a good example. But then, even though the canonicalization makes them into something else, it is usually based on upon perceived artistic importance, the Lenna is a different and arguably sui generis phenomenon.
[0] a scan of the full centerfold is included in this piece (NSFW, obviously): https://kevinrye.net/index_files/1972_playboy_magazine_the_l...