Something's missing from his marriage: Sex

You are incorrect when you assert that there are no medical reasons for this. Myriad physical and psychological issues come into play.

The way you portray your marriage -- being with your best friend, getting along well and basically living peacefully -- makes it sound ideal in many ways.

There is a high likelihood that even if you found a new partner, the same sexual dynamic would settle in if you were lucky enough to be together for almost two decades. You are fortunate to have a happy relationship to build upon.

You don't mention speaking to your wife about this. You also don't mention things you could do differently to ignite the spark and keep the flame burning.

Before you resort to making assumptions about people having affairs, you should try the universal aphrodisiac: communication. Intimacy starts there.