There’s a common saying that goes like this:
XYZ is like sex in high school, everyone’s talking about it but nobody does it.
Is there a frequently-used analogy or word for the opposite of that? Something everyone’s doing but nobody talks about it.
The best I can suggest is "Elephant in the room"
= If you say there is an elephant in the room, you mean that there is an obvious problem or difficult situation that people do not want to talk about.
Cambridge suggests it as a difficulty, but it is also more generally used to refer to something obvious that is not talked about:
An important and obvious topic, which everyone present is aware of, but which isn’t discussed, as such discussion is considered to be uncomfortable.
According to google ngram this is an idiom whose usage has risen to prominence since about the year 2000. The increase and the usage frequency are similar both sides of the Atlantic.