We all start out single ready to mingle. Teenage through 20's are widely accepted as years to have fun and experiment.
When people go out, meet others, date - most are at their best. Have to look the best, behave the best or else things likely won't work out in your favour.
Then comes the time when many people feel the urge to take it to another level, add some meaning to life and relationship. People get married, buy house, have kids.
Life changes in more ways that one.
Goals, lifestyles, circle of friends etc - all change. It's very unlikely for married couples to hang out with single friends anymore.
Life becomes safe, routinely.
Married women get more and more comfortable. True behavior and habits become normal. No need to dress sexy or wear make up at home. No need to be shy anymore, perhaps even fart or burp in hubby's presence.
After babies come, women's bodies change. Some remain fit, while most gain weight and very few are able to manage it.
Add kids to the mix. They don't exactly make environment very sexy.
Imagine yourself coming home from work, then driving kids to extra curricular activities, chores, errands etc. By the time you come home there's very little energy. That applies to both husband and wife.
By the time you're both in bed thoughts of sleep and rest come to mind before sex.
Sex can still happen anytime but there's far less excitement. Some women and men understand that sex becomes stale and try to spice up their life, while others are adamant.
When spouse's sex drives and needs don't match that's when thoughts of something extra come to mind. Some people start dating on the side, have an affair. Those usually involve feelings and emotions which eventually come to the surface and get exposed.
Visiting SP or MP is a safer option if there's an itch for that. The key is - get in, get out and forget about it. It's a supplement, not a subsitution. Those who turn it into something more get caught and get in trouble.