Dan is right, as usual.
If the husband isn't comfortable with your presence anymore, it really is best for you just to leave. He might be worried that her wanting to play alone with you is the first step towards him losing her. Speaking from previous experience, I wish that I had told my boundary transgressing 3rd to butt out before my then BF dumped me for the 3rd.
It would be interesting to know how this turned out for the couple involved. If she's kinky and he's not, I wonder how this turned out in the end.

"I wish that I had told my boundary transgressing 3rd to butt out before my then BF dumped me for the 3rd."

Why blame the 3rd, when it's your BF who decided to dump you? Do you think if the third hadn't been there, you and your BF would still be together? Are they still together today?

I can't really blame Husband for feeling that he was getting edged out, and for yanking the emergency brake, though I fail to see how Letter Writer was the one who violated any agreements. If anybody violated agreements by trying to push the envelope, it was Wife.

Maybe Letter Writer should have been smart enough to tell wife "Wait and see how Husband reacts to you before bringing me into the discussion" rather than "Yeah, I'd like that," but I doubt the outcome would have been any different, even if it hadn't been a case of the two lovers conspiring behind the husband's back. The moment it got as far as asking Letter Writer how he felt about it and Letter Writer answering that "Yeah, now that you mention it, actually I _would_ like some time with Wife all to myself," that would be Husband's cue to call a halt because he was being edged out of the picture.

Alternately, husband should ask wife for permission to find a girlfriend for himself, so he too can go find third party play that is a) to his liking, and b) doesn't involve Wife. Turnabout is fair play.

Moral of this story is: talk to your primary before asking your secondary if they'd like more from you.