Re: Is it bad enough to divorce over?
Yes it is bad enough to think about whether you want to stay with this woman. Reasons have already been outline by others. And if you are telling the truth about signing on to a dating site with friends for a laugh, then her actions are not justified. There is a very strong chance that she cheated. By the way I do understand and believe your explanation for what you did all those years ago (I have seen more than one of my colleagues, friends etc do something similar with no intention of cheating at all).
It sounds like she has a habit of easily lying to you and then when found out, lying again to provide some kind of bogus, highly questionable explanation. Are you prepared to live with somebody who contacts old boyfriends, leaves from a party with men pursuing her to spend the night at their place while lying to you constantly when you are away, and deflects you by turning the blame on you, and you constantly having to check up on her to catch her (her never feeling remorseful to tell you on her own). I sure don't think this is someone I would want to be around let alone be married to.
So the simple answer to your question is YES!
This is my quest, to follow that star
No matter how hopeless, no matter how far
To fight for the right, without question or pause
To be willing to march into Hell, for a Heavenly cause