Well, I disagree that it's the 'ultimate humiliation'. How is it humiliating if no one knows? If he were openly dating other women to spite a loving wife, maybe. But for all we know, she's a complete bitch, and he was miserable in the marriage and looking to get out. From the billboard, we know at least that she chooses vent her upset by tastelessly exposing her private business to the public. All we know about him is that he's sleeping with someone else. He might be an asshole, or he might not be.
Assuming this isn't all a gag, which it probably is anyway.