I think most people fall in love with the idea of marriage.....committed to one person, forever, forsaking all others, till death do us part. It sounds very romantic at the time and people who do marry probably have the best intentions........at the time. The idea of finding a soulmate is very comforting.
And then, after the kids come along and years upon years spent nappy-changing, domestic servitude, repetitive boring sex, or no sex at all, washing clothes, little to no interesting conversation, ironing, paying bills etc etc the dream is over, the spouse is at best, a very good friend who you love but no longer fancy and surprise surprise, people find themselves straying, or catching their partner straying.
Lots of people do remain in committed marriages but equally, so many people take lovers, have polygamous agreements, or just end up splitting to start afresh. People feel bad for having affairs, but they're too scared to end a dead marriage for various reasons.......social condemnation, security, etc etc. I wonder if marriages do have a natural shelf-life and we're biologically pre-disposed to couple romantically with more than one lifelong partner. Lots of other cultures don't have the puritan Western view of one lover. Is it setting yourself up for failure to expect total emotional and physical loyalty to one person, for the rest of your life? "For the rest of your life" is a long fucking time! I think most people find it too long. AIBU???