Does "messing around" mean I walk in and catch my spouse having sex with another person? Or does it mean just necking-petting messing around?

I'd be apt to say "sayonara," but I'm not sure anyone can really say that for sure unless faced with that actual event. It would definitely be hard to re-establish trust after something like that. It'd also be very hard to throw the towel in on a long relationship, too. I really don't have a clue what I'd do. I'm glad I'm not faced with this situation!