I can only speak for myself.
I have not, nor would I ever cheat on someone. Not because I haven't ever had the opportunity or the desire to cheat, but because I know what effect it would have on my partner. Out of respect for them I would never do it. It's also a moral thing I hold true to myself. I would not be able to respect myself if I ever cheated.
I think that's where true trust in a relationship stems from - knowing that yourself and your partner respect each other too much to put either of you in a situation that would bring pain.
Having said that, I have just found out that my husband is cheating on me. It's a deal breaker for me. The trust is gone and I don't believe it will ever come back. Plus I respect myself too much to allow myself to be with someone who could knowingly put himself in a situation that has brought me so much hurt and pain.