RB,
A funny movie that I would bet you have never heard of is "Cold Comfort Farm."
There is no infidelity. The actors are British, so if you have trouble understanding them, use your English subtitle feature. The movie may seem slow at first, but please keep watching- it gets REALLY funny if you stick with it!
"Little Miss Sunshine" is another (more recent) favorite of mine. One of the characters swears a lot and references sex, not sure if that would be a problem, but it is really worth seeing.
I recently watched "The Matador" with Pierce Brosnan- it was entertaining. No infidelity, although the lead character has several one-night stands and there is a fair amount of violence or references to violence. This is definitely a rated R film (don't watch this with your kids!).
I have to admit, several of my favorite films have at least one character who is or has betrayed his/her partner (i.e., Mystery Alaska (movie about hockey), and several romantic comedies--Bridget Jones Diary, French Kiss, Sliding Doors).
You'll laugh, but one of my all time favorites is Jaws!

The drunken sailor scene is funny, especially when you know that the actors were actually drunk when they filmed it. (Yes, I do realize my tastes are esoteric!)
I would also appreciate movie rec's from others.
Marie