I watched my dad's cheating tear my family apart. Divorce, moving miles away, pushing my mother into poverty, the works. I've seen the trouble and devastation it can cause.
But, you know what? People have it in them to try and be better. If I cared enough for them, and I had reason to believe they were genuine in their attempt to move on, I would give them that chance. You'd be surprised how people can change for the sake of someone they love.
Equally, though, you can't change some people, and you're better remaining friends or acquaintances than dating them and hoping they'll change for you if they've showed no indication of leaving their past behind them thus far.
Hell, if people were to refuse to date any of us because of the people we used to be, many of us would find ourselves without friends, and love.
You don't always know the reasons behind cheating, either. Some people aren't well suited to monogamy (I prefer it, but if I loved my partner and they wanted to have an open relationship in an attempt to save ours, I'd be willing to let them try it). Others might have been being cheated on at the same time. Sometimes it's just cowardice and lack of courage to call off a relationship because they've found someone else. Whatever.
That being said, I will admit that I do tend to have a negative impression of people who are cheaters. It is selfish, to me, and might change how much I would want to date them. But if they told me themselves (in no bragging form) I would respect them for being able to tell me that in the first place.