Yes I cheated on a BF in the past (many years ago), with one of his friends.
Why? - well initially because we were drunk, so snogged. My relationship with the BF was boring, dull, safe, staid, in a total rut and limped on for a few more years, although with hindsight this was clearly as sign that All Was Not Well then. I was young and immature. Having an affair was really exciting, I felt alive, wanted, sexy, like something was actually happening in my life in general. Having a secret was fun. It wasn't love, it really was just sex. Oh and the friend had a much bigger cock than the BF
I'm not justifying it - but people are asking why, well those are my 'reasons'. I wouldn't do it now because I'm in a totally different place and a totally different relationship and I couldn't do it to DH, and I have no desire to. But I remember at the time not really caring if my BF found out - although when it stopped I do also remember really, really hoping he wouldn't.
Actually, until DH I don't think I was ever faithful in a relationship, but that was also partly because I placed a lot of my own self-worth and self-respect on how men saw me/valued me/fancied me. I was a fully-subscribed believer of the epithet 'The best way of getting over someone is to get under someone else'.