I'm a bit unorthodox on these kind of subjects - I have my reasons, but I won't mention them here.....
As far as I see it, let me shed some light on certain things:
The human condition is not static - and by that I mean simply that no 2 people are the same, and you can't judge any 2 people equally, and people CHANGE.
Because she slept with someone else, doesn't make her nasty or evil or weak - it doesn't mean that you guys have problems in any way or that she's missing out or longing for something or that you're not doing something right or that she doesn't love you or whatever may be going through your, or anyone elses mind.
It could very simply have just been, for what it's worth - some sex. But for what, why would she, you ask?
Easy.....a rush? a change? spur or the moment? lust? curiousity? variation? etc....
Not because you consider yourself or her never ever ever doing something like that, should that mean that the same thing holds for another person.
Maybe a month from now or a year from now you'll wake up and feel the same way. Maybe one day you'll work closely or meet someone and just have an overwhelming lust for them - maybe you won't take up on it, maybe you will - it's just sex as long as there are no emotions; and despite what people say, it feels very good and it is fun, and there's nothing wrong with exploring it with whoever as long as whoever you're committed to is OK with it.
But who likes that right? What lover in their right minds would be ok with their partner making it with other people out there?
Some people are freaks, and I said that seriously....and not in a bad way. Some people like sexual variation, some people don't - some people like and treasure the thought of 1 significant other forever, some prefer to play the field for a while. It's all a matter of variation I think.
Alot can change in 8 years. Self actualization and fullfillment is a daily activity, we learn more about ourselves and our psyche up until we die basically....
I just got married recently. I would be very hurt if she were to go out there and sleep with another guy, but as long as she wasn't indescriminate and doing it all the time, I'd be forgiving because I know what it's like to love someone with all my heart but sexually be interested in, many. I mean, think about it....if she tells me, then it means something; I'm not at all aggressive, I won't hate her if she leaves me and she knows it, so what do I really have to lose, except maybe some pride? and why wouldn't she just leave unless she really does love me right? You just need to think of the positive in everything; and work things out. Now I'm not saying be whooped and let her be with 30 guys or whatever, but if it's a one or 2 time thing, take your losses and move on.
And for the record, maybe open up a little and explore. Maybe she's the type that sexually wants variety, maybe not all the time, who knows. And if you're both willing to compromise, maybe you can have variety too.
Sex is not about Love, Love is not about Sex. We're all caught up in the 1200's where people consider sex as this treasure that is ONLY to be shared with the one you marry and all that, I think this is such a homewrecking device. You know what? I have nothing against that.....and I do feel that sex IS a treasure between the one you really love and are sharing, or planning to share your life with - but I don't believe for one second that it is required to be between 2 people, and 2 people only. I for one believe other elements can be introduced and as long as there are no emotional interferances or hangups (and there won't be if your love for each other is true), then I don't see the problem. The only way I see a problem is if she's greedy - like she gets whatever, but you're not allowed to do the same, kind of things, in which case, she'd have a single person's mentality and should not be with someone. In that case, she would require personal healing that noone can give her but herself.....
I think 2 people that love each other should be able to do whatever they please when it comes to sex, as long as they make love with only each other (notice the difference, SEX and making love are 2 completely different things). It should be intimate between the 2, but fun between more than 2. I don't like the idea of open relationships or anything like that as in each person being sexually independent, but an introduction of another partner or 2 between the couple, is fine; and if compromise is present and fair then there should be no problems whatsoever.
You and her need to discuss all this.
If she can really be sure she'll be fine with one lover forever, I say forgive.
If she can't then look into yourself and see if there is an open mind enough to explore, if you don't then you know what you have to do.
If SHE has the problem exploring, but then does this again, then she's one of 'those', I refer to as eminem people, who want to have their cake and eat pie and ice cream too, and once again, you'll know what you have to do.
All this stuff can't be great for someone who is religious to read....but just consider it and stretch your mind as far as it can be open.
God loves you no matter who you sleep with as long as you're a good, kind person, and love and worship him; otherwise, well - we would ALL be going to hell.