He revealed he avoided putting a label on his relationships to avoid "technically" cheating.
"I think what happened for me, you go through a lot of your life in your teens and I was younger than everybody in high school and college, and so I just didn't get a lot of girls. When I started to get that attention, I loved it.
"I escaped 'technically cheating' by keeping my relationship ill-defined, but it was really cheating. I definitely was dishonest and selfish … It was happening before I was famous."
So, did his behaviour change when he met his wife, model and cookbook author Chrissy Teigen?
Legend said on the podcast episode he and Teigen "hit it off" immediately when they met.
"We were both seeing other people at the time. There was kind of a looseness at the very beginning, but we fell in love not long after that."
He says he realised he was happier being honest and faithful with one person.
"At a certain point, I just decided that person was Chrissy. I decided I wasn't gonna mess with somebody else anymore."