What's Wrong with Infidelity?

Seth and his girlfriend of many years were already engaged when he discovered she had cheated on him. It was only once, with a co-worker, but the betrayal stung. “I had jealousy, insecurity, anger, fear,” he recalls. “It was really hard to talk about it.” He wondered whether his fiancée's infidelity meant there was something fundamentally wrong with their otherwise loving relationship. He worried it was a sign that their marriage would be doomed. He also still felt guilty about an indiscretion of his own years earlier, when he'd had a one-night stand with an acquaintance. “I knew that what I had done meant nothing,” said Seth, a New York-based entrepreneur in his early 30s. “It felt like a bit of an adventure, and I went for it.” But anxiety about these dalliances gnawed at his conscience. How could he and his fiancée promise to be monogamous for a lifetime if they were already struggling to stay loyal to each other? Did their momentary lapses of judgment spell bigger problems for their union?