When someone cheats in a relationship the consequences can be devastating. i speak from experience. Opening yourself up enough to trust someone with your innermost thoughts and feelings? That's taking a big risk.
To find out that this risk hasn't paid off can be absolutely crushing (cut to memories of me, a liter bottle of Boone's Farm and Bridget Jones' Diary on repeat). Not only does it cause you to doubt your partner's past actions, it can make moving forward almost totally impossible.
Many marriages don't survive one member straying. Others survive and, with hard work, go on to thrive. You can't change how you've behaved in the past, but you can change how you behave in the future. Just because someone violated your trust in a past relationship, this doesn't mean your new partner will do the same.