I think you should sit down with her and tell her exactly how you feel. Tell her it hurt you to see her kissing someone else, and say that it hurt you because you love her.
Try to keep it all calm if you can. I don't think it's grounds for a divorce, but you need to talk it over as adults. She needs to understand that that sort of behaviour is not acceptable, at any time of year.
Maybe you did overreact at the time, which could have caused the argument, but maybe she should try and understand that you only reacted in that way because you love her.
Try and get her to see that if you hadn't reacted at all, then it might have seemed like you didn't care about what she did, and that sounds like a far worse relationship to be in.
