Hormones aside, it is your brain that makes the decision to cheat. I can't say what I would do because DH has never put me in that situation and I hope I never have to find out what it is like. I trust him completly but anyone is capable of anything.
You have to make the decision of what you can live with, if you can trust again and bear in mind the consequences of any decision you make regarding the relationship. If someone else brings another into the relationship via cheating, whatever, there are probably huge problems in the relationship on many different levels. I have done the long time apart thing and it's hard. There is no one there for support, companionship physically or mentally, ect. But this is something that will make or break a relationship. If you cannot handle time and/or distance from each other then chances are you are not going to make it long term. Marriage is harder than anything you will ever do but the rewards are so worth it. There will always be a "reason" why someone cheated but the decision utimately was theirs and they made it. Unfortunately someone else usually will bear the brunt of the pain due to that decision. Trust is a delicate thing, it can be damaged so easily but not so easily fixed.
But yes, the trust for me would never be the same if I were the one cheated on and the person would be different in my eyes. If I were single, he would be gone, end of discussion. But when you bring marriage and kids into the picture it complicates things, but that is my opinion.
Yes, people make mistakes but this is the mother of them all next to physical/mental abuse. The flesh may be weak but the heart and mind should be stronger.