No, not all men cheat. A lot of men encourage this lie though, so they have the lowest possible expectations put on them. That way, them doing something simple like being faithful gets them god status when it is literally a minimum requirement of a marriage. If you ever hear a man say anything similar to this "all men cheat" thing run the other way. They are just trying to prime you to accept the abuse they have planned as normal.
I am always curious about the flip side though. The women who push the all men cheat talk and really believe it. Why do they even want to be in relationships, let alone a marriage? What would be the point if in your mind you've already accepted that you're going to be cheated on? I wish I could ask but I don't really know any women in real life who believe this, or at least are willing to admit it out loud since most people would take it as a display of low self esteem.