Agreed that people are not smart about who they get married to in the long run, but I believe that's because people don't THINK about the long run. I know very few people who get married for practical reasons like insurance. I believe most people get married for entirely unpractical reasons and think those reasons will last for 10, 20, 30 years.

For instance, I think sex is important in a longterm marriage, but some people seem to stake their entire relationship upon it. If the sex isn't great right off the bat, most of my girlfriends ride the guy off, or if he isn't their jaw-dropping "type." It doesn't matter if they have great conversations or if the guy is in a stable job. Just from my personal observations, people seem to think marriage should be exciting, stimulating, and sex-filled 24-7, instead of a roller coaster of good times, bad times, hard times, fun times.

Personally... I think affairs are mostly narcisstic (and I'm quantifying affairs as sex outside a monogamous relationship without the partner's consent.) All about satisfying MY need (sexual variety, youth recapturing, whathaveyou) over YOUR need (monogamy, trust, etc.)