Hi Reen,
It is possible that you will not see this, if you have gone again, but I will post it anyway.
In one of my earlier posts, I said that just because something could have happened does not mean that it did.
I stand by that, and I hope that it will help you to see that what happened twenty years ago may genuinely have been an evening out dancing where nothing serious happened. Sure, it should not have happened at all, but the fact that it did does not invalidate the marriage that followed it, and it does not invalidate your wife as a person.
No-one is ever comfortable with their loved one going on a 'fun' date, even if it is just dancing, because we are all prone to nagging doubts, and the horrible power of 'What if?'
To counter that, it is worth considering that your girlfriend is not a good liar. When you spoke to her on the phone back then, you could tell that something was off. It seems likely that if she was actively misbehaving, you would have detected it immediately. And all you have from that period is one incident.
Maybe no-one would award your wife a merit badge for heading off to a concert with a guy, then abandoning that and going dancing instead, but it really is possible for a woman to spend time with a man without having sex with him, or crossing any big boundaries.
You know your wife, and if she was the kind of person who would sleep with someone she barely knew on a first date, I credit you with enough intelligence to have detected that in the twenty years that you have known her.
If she was 'that kind of gal', she would have done this repeatedly, and you would have caught her.
You picked up on something being 'off' in that phone call, but you have not picked up on anything similar in twenty years. That tells me that either your wife has become a proficient and callous liar, or she has not done anything that she should not have done for two decades.
Reen, your wife may not be a perfect angel, and I am sure that you must have your faults as we all do, but between you, you have a good and real marriage that you both enjoy, and which I believe you should continue to enjoy.