Yes everyone has fantasies, but what makes this a more serious one, is that your wife has contact 3 to 4 times a year. The fact that she has denied it, then admitted it, then gone back to search the guy out on the internet and intentionally erase the history, makes it serious. Thoughts can become words, words can become action, if the thought is not pursued, it's less likely to happen.

You make a valid point about lust, both men and women have it, but once emotional involvement comes into play, which has a stronger potential of occurring in your wife's case, if it hasn't already. Is normally detrimental to a marriage.

My advice is to seek out a Marriage Counselor, alone first and see what advice s/he can give in order to bring your wife to the session.

Probably both you of will have to sit down to have a serious talk, about going to therapy, convey it as an intent to improve the marriage, that there are faults that you have (twodog74) and that you need a counselor to improve. (Before you say or do this, check with your counselor first).

And Good Luck