[Borges]

Extramarital activity of unmarried adults is their own business.

That's what I meant actually. Adultery can be defined as any sex outside of marriage by anyone.

[annalex]

“Adultery” is an affair in which a married person is involved.

“Extramarital sex” is an affair between poeple not married to each other. If neither is married to anyone else either, we have extramarital sex but not adultery.