It's a loving spouse's worst nightmare: standing by, helpless, as your partner seeks sexual attention from others-a discovery that quickly turns to shame and regret. If only you were prettier or more exciting, maybe you could keep his attention. If only you earned more money, maybe she would be attracted to you.

But the truth is that, for many, online and real-life infidelity is an addiction-a compulsive need for other sexual partners, as intoxicating as any bottle or pill. And with the rise of websites and smartphone apps, getting the next fix is often just a click or screen tap away....