When is information, TMI?

Question: When is information, too much information? I realize that this is a very subjective question because TMI for one person is not enough for the next. I consider myself a very modest person, at least when it comes to the amount of info I put out there for people to scrutinize. Many people say that is why I am successful in my role in business, which requires a great deal of trust and keeping informational secrets. However, when I open myself up a bit (usually after several cocktails) I have my friends squealing with delight, and occassionally disgust. I have a few very close friends (like I can count them on one hand) who "really" know what happens behind closed doors at my condo. Most people get a very "G" rated version of my escapades. Which brings me back to my question.

I was recently exchanging dirty boy stories with a good gal pal of mine, kind of like a "if you show my yours, I'll show you mine" banter, only with no show, only tell. When I suddenly let it slip that I have been occassionally seeing a guy who has a partner of five years, I thought she might kill me right then and there with her Jimmy Choos. I quickly realized that I crossed the line of TMI. However, this is someone whom I have known for well over a year, and when I never got even a cringe of disgust upon mentioning the goings on at an underground leather bar in NYC (one time visit on a dare only fellas, sorry!), I figured the fact that I was enjoying the occassional company of another man, albeit a taken man, was a walk in the park. How wrong I was. So how do you know when too much is more than enough, and hardly anything is just right?