A Man Fact is the sort of fact that men have a tendency to profess as the truth and as though it were an actual fact but in reality they actually have no idea and are making, at best, an educated guess.
Woman: I really don't think you should have another beer.
Man: I disagree, the most recent study shows that regular beer drinking can improve my mental capacity for learning new tasks. I am sure you would appreciate this in the bedroom honey.
Woman: That's just plain untrue. That is a Man Fact.