it doesn't pay to lie

“Kathy almost divorced me once,” I said. “We had been married for ten years, and I had an affair with another woman. When Kathy found out, she was furious.” 

“Why would she care that you had sex with someone else?” Jane asked. 

“It wasn’t really about the sex,” I said. “It was that I lied to her about it. Having sex with someone else only counted as a hormonal weakness in her book. Lying counted as disrespect, and she didn’t want to be married to someone who had no respect for her.”

—John Scalzi, Old Man's War

At the same time, I'm pretty sure I'd be furious if I caught my wife having sex outside of our marriage. That's not just "hormonal weakness": that's commitment weakness. Lying is indeed a form of disrespect because it means you don't trust someone enough to tell that person the truth. Love is nothing without trust. But lack of commitment is also a sign of disrespect: it means you're looking around, seeking an escape hatch or a back door, which again implies a lack of trust. How does your lack of commitment make your partner feel better about you? How does it make her trust you more? It doesn't. Sex means nothing for a lot of people; it's just a way to have some harmless(?) fun. For two people who understand that sex is just part of a larger, playful, selfish game, well, I guess that sort of mutual understanding means no harm, no foul. But for some of us traditionalists, sex is a sign of deep intimacy and commitment; it's not something you just throw around like garbage.