If you're going to participate in sexual activities that you don't want your teenager to find out about and use the internet as part of participation of said activities, don't share a computer with them.
Dear Prudence,
My husband and I are in our mid-40s and have been married for 20 years. We have a teenage son and a younger daughter, and for the past five years, my husband and I have explored swinging. We've had sex with two other couples over the past several years, and find it a fun way to be both intimate and adventurous together. We currently meet with a couple once or twice per month when we go to a hotel and leave our children at my parents' home. We’ve been careful and discreet but little while ago, one of us forgot to sign out of the account we use to contact this couple, and my son found a sexually explicit email from the other woman that he assumed was directed only to my husband and concluded his father was cheating on me. He's confronted my husband, who was flabbergasted and said little apart from unconvincing denials and sputtering about privacy. My son threatened to tell me. I can't imagine that coming clean to him in any detail about our private lives would be healthy. I’ve been trying to come up with a believable lie or half-truth that could be told. What do we do?