Nature has played an unkind joke on humans. We're evolved to be semi-monogamous, and for most of our evolutionary history our life expectancy was fairly short (mid-20s for most). Now we live for 70+ years and the Disney fantasy of happy-ever-after doesn't match with reality, so we attempt to deny reality. The sad thing is, in most cases it's not sexual infidelity that causes the harm but rather the lies that are utilized to hide the infidelity. That's why folk who practice ethical non-monogamy (in which there are no lies and no hiding) tend to be happier and healthier. But, alas, most people lack the emotional maturity necessary for honesty, even when the topic has nothing whatsoever to do with sexuality. Lies are everywhere in most relationships, and that's the problem, not affairs per se.