Cool, "intermarital sex". I think it's just been coined, but I really like it! I'm not even sure the consent part makes it inter vs extra. Would sex with other people in an open marriage be inter or extra? Perhaps Mr. Alura gets to decide since it's his word.
Spoo: I think the easiest way to remember the difference is the two examples you just gave. Intramural sports were within the same school. Interstate highways cross state lines...