A Reader writes:

My gf and I have discussed this considerably.
Without condoning it, I have concluded that it’s
certainly not different in principle than slogans on
the chest area of tight t-shirts adorned by a woman
with a bosom. The only key difference is that the
latter (ie, sloganed t-shirts, not women with bosoms)
have been around since I (born in 1968) was too young
to question it, and the former is a recent invention.
For someone much older than I (and, I guess, you), or
much younger, there’s probably no perceived
difference.

Me: I’m not entirely sure I buy this, though it makes a good point. The difference, I think, is that in the 1950s and 1960s men wore shirts that said stuff on them and women were just following suit. I am unaware of any men who wore butt slogans — at least not before women started it.