Here is a possible way of looking at the timeline. This is sheer speculation, but it has been on my mind for a while so I'll just put it out there.
Way back in the day, when the two of you were living together, you weren't exactly on the same page in terms of the level of commitment and seriousness of the relationship. This often happens among young people who are dating but not married, by the way. More specifically, it has been my observation that it is an issue with boyfriends/girlfriends who move in together.
Specifically, she was not as committed to you as you were to her. She still saw herself as essentially a single person, with a guy she was dating who happened to live with her.
Like many young dating single people, her boundaries were fuzzy and she said "yes" to sexual adventures. Many of us did this kind of stuff before marriage.
In other words, her sex with the other man was sort of a hybrid of "sex stuff our wives do with other men before they enter into relationships with us" and an affair. Married people make express promises of lifelong fidelity. Boyfriends/girlfriends in their early 20's often don't discuss anything very clearly nor expressly; they tend to rely more on individual wishes coupled with assumptions. We know what value "assume" has when it comes to the heart of another.
In her case, though, she realized at some point that she was falling in love with you; and then she did in fact fall in love with you, get married to you, and start a family with you.
If that is the case, it's likely her reasons for not telling morphed over time. At first she didn't tell because she wasn't serious about you. She didn't respect you enough to be honest. She was cake eating in its purest form. You've described her as very pretty. Very pretty women in their early 20's have often lived lives of having the red carpet rolled out for them by every man who has ever been in their presence. Their experience with men is a pack of panting hounds eager to eat a mile of their shit just to see where it came from. They often have never been called upon to exhibit any form of respect for a man (and, to be fair, men have shown them very little genuine respect at all -- instead, their life has been a fire hose of obsequious assholes spewing ersatz concern as a ruse to get into the panties).
Later, she didn't tell because she was falling in actual love with you and didn't want to bollix it with messy stuff from the prior "just dating" phase.
Later still, with marriage looming, the thought of derailing what for her was a true love/truly in love, drove the truth underground.
Eventually, though, it ate at her and as she realized you were a stand-up guy, in it for the long run, she felt you deserved the truth.
To me, in a way, it is the dishonesty more than the sex that is the issue. As mentioned above, her fling with the coworker is rather plain vanilla for a sexual fling by a person in her early 20's. It's almost of a species of the category of "sex stuff people do with others before entering into the relationship with the spouse." I'm of the school that people contemplating marriage should discuss their sexual histories with each other, because it will come out eventually and it's important to know if there are skeletons in the closet that the other partner might be uncomfortable with, that might affect the decision whether to trust the person with a promise of a life.
This was a big skeleton. Not only did she cheat on a boyfriend while living with him, the boyfriend she cheated on was you. An honorable person would have been up front about this from the outset. It's possible you would have left her had she told you, but it's also possible she would and could have won you back had she wanted. Boyfriends/girlfriends do this all the time.
By way of counter-example, the woman who is currently my wife flat out told me, when we first started dating: "This isn't exclusive for me." I knew she was seeing another man already at the time she started seeing me. Later, as we got serious, we agreed to be exclusive and I respected her for her brave honesty on that bit. It made me trust her easily.
By the way, my wife is very pretty too. She is not the first very pretty woman I've been with. Several of them have been very direct with me about being non-exclusive. It seems that for a pretty woman in her 20's, sex can be a true smorgasbord, and most of them learn that they generally won't lose a man if they tell him they won't be exclusive with him. In other words, a woman like that does not need to learn to experience empathy. The world does not require empathy of its pretty young women.
So you have a wife who was actively dishonest, and she was thus for self-serving reasons, and she remained dishonest for years, despite your repeated asking. This is the troubling part. An element of cowardice, cake-eating, lack of backbone.
The fact that she finally did tell you is a credit to whom she has become. I'm gathering she has matured and grown in perspective and empathy.
This could be an opportunity to re-set the marital relationship if you think this is something you desire. Are there aspects of the marriage you are unhappy with? For example, would you like her to initiate sex with you more often? Or are there sexual things you'd like to do that she has said "no" to (like blowjobs in the shower)? Or is there some other aspect of the marriage where you feel she has been less than giving?
[This message edited by Butforthegrace at 5:00 PM, November 6th (Tuesday)]