Did adultery used to be illegal in the US?

Yep, this is true. In fact, some states might still have laws on the books that make adultery illegal, but they're not enforced today. They haven't been enforced for at least 30 years, I believe.

Also, all but the very sleaziest hotels would have a "hotel detective" whose job it was to make sure that there was no hanky panky going on. He was always on the prowl, making sure that single men and women who checked in were not entertaining someone of the opposite sex in their rooms. In the sleazier hotels, of course, there was no hotel dick (no kidding, that's what they were called) employed, and a single man in search of female companionship could usually give the bellboy a tip, and ask about any lonely women.

Read some of the older mysteries, especially the hardboiled mysteries, you'll be amazed.

Probably. Adultery laws are still on the books in some states (Georgia and Illinois come to mind; I know there are others), even though they're rarely enforced these days.

Yes, and still is in some states, like VA where sex outside wedlock, which is also considered adultery, is illegal.

http://divpat.org/Married-Girlfriend-Is-Pregnant

http://www.squidoo.com/PriceOfShackingUp

yesss. always have been.

People used to be very religious and it was one of the ten commandments in the bible. So it's a sin if committed.