Well my sister has never cheated on her bf - and I certainly don't think her bf has either. I've met the guy before - he's the sweet, shy, adorable sort - I certainly can't imagine him cheating on my sister, considering he stayed in England for her! He's in Wales and my sis is in Scotland.
You could say I'm "technically" conducting a mini LDR cus my bf lives in Germany, so I won't see him til Sep when he comes back for uni. And no, neither of us has ever cheated on the other. He would tell me if he did - I know the guilt would eat him up that he will HAVE to tell me if he ever did cheat on me.
My parents stayed apart for a year, one in England and the other in HK, and neither of them cheated on the other. In fact, I think our neighbour was interested in my mum before dad came to England too, and I still remember, my mum asked me, when I was 9, if I could stay in the lounge with her when the niehgbour visits for a cup of tea. She didn't want to give him any opportunities to attempt anything, and I know she avoided all opportunities to be alone with him.