As Bugsy says, can you ascertain if he is chatting or flirting? is it the equavalent of mumsnet? I think you are within your rights to ask him what sites he uses and then say you might visit the chat rooms yourself sometime. After all if he was out for a night with friends, wouldn't he tell you where he was going and if you surprised him by turning up unexpectedly, he wouldn't be horrified, would he? After that... hmmmm ....I just don't know. Just as you wouldn't open and read his post, can you expect him to let you read everything he's put on screen - and if he did, would he then feel he had a right to look at all your postings?
Taking out of its virtual context. Does your husband have many femail friends? are they old ones dating from his single days before he met you, or new femail friends? What's going on in his life right now? has he sought out lots of femail company recently?
And some people just like making friends. I'm married to one. It has been known for a woman I am only slighty acquainted with to phone us at home, say a quick 'hi' to me, ask if my husband is in, and then invite him out for a drink with her and her friends. This type of thing has happended a few times in the last 10 years. Tests my patience a bit, but I keep saying he's just a very sociable and, as far as I know, extremely loyal person.
I don't think you are being oversensitive, though, especially as he didn't tell you about his friends - you discovered it. I think you have a right know who he is spending his time with, virtual or not, and agree with him on ground rules about what you do or don't say on line, and to whom.