I understand, and yes, you should defer to your atty's advice on this. But can't you BLUFF it to OM, by creating a contemporaneous communication with your atty and, say, another 3rd party letting them KNOW it's a bluff? e.g.:
(e-mail to atty, cc'ing another trusted friend and a web-based e-mail account):
"Today I decided to make OM think that W and I have had sex recently, in a last-ditch effort to try to save my marriage. Even though we haven't ML since (date), and of course I CAN'T even do that because -- as you've advised me, it would indicated a legal acceptance on my part of her affair -- I felt like it might drive a wedge between them if he THOUGHT that I had, and I do still want to try to save my marriage if I can."
