Some of the characters in "Magnolia" are agonizing their infidelities, cheatings and betrayals - not because they got caught - but because their conscious is eating them alive. One character reaches the point in which she can't live with herself anymore. It doesn't matter how much time one manages to suppress and silence the haunting thoughts; they will eventually come to get the best of him. And even if he's managed to live in total denial - they will grab a hold of him on his deathbed.
People that are involved in such acts will feel extremely uneasy watching this movie, people that are on the brink of engaging in such acts would probably think twice, and people that are not even in the surrounding areas - for the time being - will try harder to fight and avoid them.
If the above materializes - the movie has fulfilled its goal.
There's a beautiful and amazingly fitting song near the end of the movie, "Wise up", and it's chorus is sung by all the characters in an overpowering climax: "It's not going to stop, it's not going to stop till you wise up". In the context of this motion picture - its significance becomes crystal clear.
Do a little research. It shouldn't take you long to find a dozen or so newspaper articles documenting frog storms.
". . .but it did happen."