I have similar story to Philip's.  My husband began late night chats with a "friend" that he didn't know.  She is 16 years younger.  Anyway, things progressed and I gather they would "chat" late at night while I slept.  From everything I have been able to put together, they "friended" each other in mid-April.  I'm not sure when the chatting started, but by the end of July the few communications that I found indicated that the conversations had begun to get sexual in nature.  Of the communications that I discovered it was evident that my husband had tried to pull away a couple of times, but for whatever reason the relationship continued.  I found some emails that were sent early in Sept that were close ups of her various body parts.  I was not able to find a return communication from him in regards to these, but that too, was probably done thru facebook chat.  Late in Sept she emailed about his not corresponding with her.   I didn't discover any of this until just 2 weeks ago, when I inadvertently discovered one of his email accounts open on my computer.  I decided that this warrented further investigation.  On Feb 12, late in the evening, I replied to one of the photo emails, knowing she would think it was him.  She responded that yes, I should recognize the body part in question.  Now i knew I had to know more, because it was apparent there was more to know.  But, I decided to express my love for my husband for Valentine's day like I hadn't done in years.  Monday evening, while my husband was away, by what had to intervention from God, I easily cracked the password on my husband's computer.  I got into his primary email and discovered a 2 incidences of communication.  One, dated in early November, where she tells him that she can't wait to see him.  The next set were found to be dated early Feb, where she questions why after they had gotten together in November that he ad pulled away.  He explained that it had nothing to do with his disappointment with her, it was just that he had too much to lose and didn't want to do that.  I confronted him that same evening at the same time that I sent a email to her letting her know what I had discovered. 
My husband was mortified, and I believe he truly regrets what he did.  That doesn't make the hurt go away, and the thoughts that run through my head constantly as I imagine the conversations that took place and progressed to the point of infidelity.  I'm ok when we are together, but any time we are apart the thoughts and visions begin-when will it stop, and when will I trust him again.  By the way, I told him that all communication had to stop, and was to unfriend her (he did), and he gave me a list of all of his user ids and accounts to all but 1 email account (he says he never uses it and will have to look up the password).   Right now, I pray for peace in my heart!!

usedtobe,

If you have any inkling that your relationship with this man is starting to go in a direction where (you feel) it doesn't need to, then you need to just end it. Some of the signs are him opening up and telling you negative things about his marriage. How his wife is terrible at meeting his needs (any of them). And if he is looking for you to lift him up in any way, like you saying "Well, that's not true! How can she be that way toward you? You're great." Or anything like that, then the relationship has crossed a line.

The fact that he is married needs to be the #1 reason you end this....now. You're feelings are probably right on this one and I know you don't want to be the reason he "leaves" his wife. It's so easy b/c it does start out innocent, but a line quickly becomes visible and if you think he's there, then you need to walk away.

But all of this is just my opinion. I may be way off, or I may be right on....any other thoughts?