ch76, I'm pretty sure they wouldn't have been laughing at you; these things just seem to happen in rehab, and you won't be the last SO who found out their partner had cheated. Rehab seems to be the perfect environment for hooking up, in that you have unstable emotions, an atmosphere of intimacy, and probably a dose of boredom.
I know it's got to hurt a lot, but if it helps you move on, good. It gives you a chance to work on yourself.