Originally Posted by DrunkenPorcupine
It's really, really hard to be honest with your partner when you don't even know what you mean or want as an individual. It's just that many people default to monogamy so you see people who are all bottled up also often defaulting to monogamy.
People default to monogamy, to the religion they were raised with, to the eating habits of their friends, to the political opinions of the media... It's a lot of work to figure things out for yourself, and most people don't see the value in it. Then when one of their natural urges comes along that runs counter to their adopted values, they run into trouble. They want to satisfy that urge, but they also want to remain consistent with their values. So sneaking around and/or lying are the only options, outside of changing their values, which usually requires a lot of thought, and possibly some major life changes. Way easier to tell a little lie that they can rationalize away.