Every person is different on what they can forgive in their SO. Some people are fine with something that would be deal breakers for others. I know someone who's boyfriend is a real shithead and likes to call them fat in front of people to embarass them. For me the guy would be history, for them it is part of the relationship.

For me cheating is an absolute deal-breaker. There is no way I could ever forgive, forget or have any desire to do either. The trust in the relationship and the respect is the most important thing and I could never go back.

Many, many women have worked through infidelity with their husbands and have gone on to have reasonably happy relationships (although I imagine the women would always be nervous).

More than both are the women who have been cheated upon but never know. How exactly did you find out about his infidelity? A couple of points of thought for you:
1) I highly doubt that this just "happened", I would stake more than a quarter on the fact that this was a premeditated and eagerly anticipated interlude. One does not just end up in bed with a coworker without having had thoughts, desires and planning for a night when you would be out of town.

2) What is worse to you, him being willing to betray you for a meaningless roll in the hay or him having a relationship with another behind your back? I can't decide what is worse. Also consider the other type which is the serial cheater who does it regularly for the excitement. None of these types are real appealing but it might be good to examine his reasoning.

3) Doing it in your house is truly an exercise of disrespect to you. He knows that is where you sleep, where you wake up next to him, where you have talked. If he can cut off any emotion about having another woman in that most private space, I would question his feelings about all sorts of things.

4) Sex without protection is common but is exceptionally disrespectful in this situation since he lacked any concern about potentially exposing you to deadly disease. But also ask yourself if the disgust is also with a condom acting as a physical barrier and that it is more cheating because he touched her in that way....?

A good point was made: was the cheating a result of a breakdown in your relationship? Had things been going astray, had their been a loss of intimacy? My understanding is that cheating because of problems are generally the types that can be worked through. If he was acting like all was normal, everything was A-Ok but went out with someone else as soon as you left town...I would seriously take a look at why you are staying with this guy.

I am supremely sorry that you are going through this. It is a horrible betrayal and assault on your heart as well as your self-esteem. Please keep in mind that people cheat on young and attractive people as well.