Why do they call it "adultery," when it makes them act like children?

Loving Couples (1980)

Directed by Jack Smight

Walter is a successful doctor, and so is his wife Evelyn. Wealthy and good-looking, they should be a happily married couple, but they're not. Walter is so caught up in his career that he fails to keep up with Evelyn. So when she falls for the charms of Greg, a real estate Romeo, it takes Walter by surprise. Even more so, because he finds out about it from Stephanie, Greg's deserted TV weather girl girlfriend.