Back in the olden days, in 1971, we were receiving sex education in school.
The subject of STDs came up.
Our teacher told us that two children in a family developed some rather nasty sores in their eyes.
At first the doctors could not figure out what was causing this problem, until they were able to grow a culture and found that the children had developed an STD in the eyes.
How had this happened? One of their parents had had an affair, got an STD, infected their partner and the children had used the bath towel that the parents had used to dry their faces, getting the bacteria in to the children's eyes.
The biology teacher's point had been that it was, in some circumstance, possible to catch an STD other than with sex.
Me being me even then thought: "Hang on! If one of their parents hadn't decided to have an affair, they would not have infected their spouse or their children.
But of course, 'discrete' affairs hurt nobody, right...?