Yes I have cheated with previous partners before my DH. In fact I was rarely faithful in my 20's.
As soon as something became long term and serious I would end up cheating.
I think it stemmed from my first ever relationship. I fell head over heels, and looking back the first and only time we had sex it was what I would now call date rape, he belittled me and semi forced me, and I lost my virginity and got pregnant in the same evening. I then found out he had several girlfriends, around the base in which he was stationed. He was writing to a few girls in our home town, who were old school friends of his, bragging about how cheap the prostitutes were while on a six month exercise abroad. I had a part time job in the same place as one of the girls, and overheard her laughing about it, and me in the staff canteen one evening. All the while I was going through planning a termination alone as I was too scared to tell my parents, finishing my a levels etc, and writing him soppy love letters, talking about how much I missed him to everyone.
I think after that a preservation mechanism sort of set in, I'll cheat on you, and then if you do it to me, it won't hurt as much. Sounds ridiculous now, but I was young. Usually I cheated with the next person I would have a relationship with. I wasn't chasing down other peoples husbands, I knew the people I cheated with.
I got it all out of my system by the time I'd hit 28-30 and then spent a lot of time on my own enjoying my own company, learning to respect myself and calming down a lot.
I would never have even contemplated marriage at that time. I would never cheat on my DH even though temptation has been there (I work away a lot, he would never find out). I thought long and hard about marriage as a commitment before we got hitched.