They do offer an "Affair Guarantee" where you pay $200+ and if you haven't had an affair in six months then they give you your money back. Yes, people in open relationships could use this as well, but the whole modality of the site is specifically about cheating, not just being a marketplace for open couples.
I know a guy who worked at Bletchley Park, and he said that they could never crack the luftwafe code because it was a true OTP implimentation. The pilots were literally issued a little one-time pad before flight, with letters on a grid of co-ordinates, and then instructions sent from ground ops would simply be pairs of x/y co-ordinates so the pilots could just look at the pad and see the message out. For each new message they would tear off one page and have a new arrangement of letters.