I think defining terms can be important to understand our emotions.
Frankly – based on what you share and I acknowledge that it might not be enough to be 100% clear on this – then the infidelity you are experiencing is "limited" to that kissing event.
Note I place "limited" in brackets because it IS infidelity. It is definitely 100% wrong and can cause just as much damage to you and your marriage as any infidelity. I am NOT minimizing it. But it’s not six events plus the kissing. Short relief – its like only being punched in the nose and not kicked in the balls too. If anything it might shorten the time you need before you can respond.
Infidelity can really only take place if people are in a relationship with expectations of certain behaviors or actions, and those behaviors or actions are broken. We here on SI are mostly about sexual or emotional infidelity, but domestic abuse, financial shenanigans, maybe even all sorts of addictions… are infidelity per se.
We can have all these discussions about if dating someone else n-days after breaking up with someone else is moral, sensible or good. But fact is – if one or other is clear to the other person that you are not in a monogamous relationship then its not infidelity.
This does not mean you can’t be or shouldn’t be traumatized by the discovery.
But call it by its correct term, and IMHO that term is NOT infidelity of the type we tend to deal with.
It’s possibly lack-of-trust, secrets kept, lack of disclosure and/or not giving you the correct information to process if you should have started dating your then-gf again after a x-day break during which she dated Sam or Max or whatever. It’s definitely a reason to stop and think. It definitely could have impacted a decision to resume the relationship.
MAYBE – just MAYBE – you have some accountability. Like if someone tells me "We are over" I would understand that phrase as the relationship is over. MAYBE you used that phrase with some expectation that "over" was a temporary thing. Or maybe you understood that phrase coming from her as a temporary thing. Maybe the issue could be unclear messages or misunderstandings. Not accountable for her having sex with someone else, but maybe accountable for it not being 100% clear if the relationship was "over" over or only "over" "over". Clear messages.
Or maybe I am way off.
Once again: NOT minimizing you pain.
It’s just that I think it could help you a lot to see the problems for what they are.
We have posters define your wife as a serial-cheater and shouting at you to leave her.
If you don’t want this marriage, you don’t need any excuse to leave. But do so on the right grounds. Do so because YOU want to leave. There is a certain difference in the following two statements, despite them basically saying the same and having the same end-effect:
"I divorced my wife because she kissed another man and I also just discovered that before we married she had dates with six other men during periods when we were on a break"
"I divorced my wife because I have issues with her having kissed another man and not telling me about the six dates she had during periods when we were on a break before we married"
Never forget that you decided to marry. Didn’t have to justify it to anyone. You just decided to marry. Basically that applies to all the time you are married. You are still married because you decide to remain married. The same applies to divorce. You don’t need a reason beyond not believing this marriage has no future.
And please – don’t hide behind things like cant afford it, bad for the kids and all that. Those might make a good case for trying to reconcile, but a terrible case for remaining unhappy.
But then… I don’t think you posted in the reconciliation forum of this site because you want to divorce.
This is what I would suggest.
Sit your wife down and tell her your concerns.
Don’t attack her, don’t expect her to defend herself. Don’t expect her to justify her actions or whatever.
I would explain how the kissing event set things off. How they triggered you. In your shoes I would accept that the poly strongly supports her truth and I would make that clear to her, but at the same time acknowledge that there is some doubt that is more your issue than hers. Make it clear that this is infidelity, and it traumatizes you just as much as had she been more intimate with OM.
I would also address the six dates/events from pre-marriage. She doesn’t have to justify them – don’t you two go into arguing if you really were over on the 14th of April 1998 or whatever. Its clear that there could have been some misinformation or unclear messages. What remains is that IF you had known then MAYBE you would have ended your relationship back then. You feel as if you maybe married on the wrong premise. You don’t know IF you would have married her, but she denied you the possibility to make that decision.
You can admit that this might not be fully rational or logical, but it is what you are dealing with. It’s the emotions that are strong in you.
Then ask her if she wants this marriage to work.
Tell her that you do. (and I’m assuming you do).
Then ask – "What can WE do to make our marriage work?"
What ideas does she have, what goals do you both share, what actions can you both take, what rut can you break out of… Basically – instead of fearing each other and fearing confrontation then embrace being honest to each other. Stop trying to win, but rather you two try to make the marriage win.