Thanks for the comments - I'm very new to this.
The main difficulty I see is that they have not (as far as I know) come to poly under their own steam.
I see a significant distinction between:
feeling that you are poly / have always been poly and naturally want to be open to exploring other relationships if the opportunity presents itself (and are happy for your partner to have the option of other relationships too, if they want)
and
already having met someone who you want to be closer with or even become lovers, and then asking if your spouse would be okay with this.
I had been with my last mono partner for the better part of a decade when I met LO. I was immediately honest about the feelings I had for this new woman. We discussed opening our relationship as I felt unwilling (unable?) to shut things down with the LO. After a few months of discussions and plenty of agonising, my mono SO decides that poly isn't for her. Two issues. Jealousy. More fundamentally, the belief that one can only "truly" love one person at a time, and that her place had been superseded by my new love interest. Our relationship ended shortly after.
The strange thing is that in the past I'd discussed potentially having a poly nature - flirting with other women was par for the course with me and tolerated by my SO - as were "close" friendships with other women provided no sex occurred - and we had discussed potentially opening our relationship at some future date. We'd even discussed the possibility of swinging. She always felt very secure in our relationship. My limerence (degree of feelings and persistent, involuntary nature) robbed her of that sense of security.
Been thinking more about my LO's husband. Think he wouldn't be adverse to discussions on poly - he's an intelligent and reasonable man. Likely wouldn't morally approve. However, I think if he thought his wife brought up the subject because she has feelings for me and wants to take it further he'd be sick about it. I don't take that lightly. I read CTF's thread (
http://www.polyamory.com/forum/showthread.php?t=75411) and I wouldn't want any part in putting anyone through such suffering.
In some sense the betrayal is complete by a wife even asking her husband if she could see another man.
Not sure how to proceed. This is a minefield.
Agree that I shouldn't be considering any discussions with her husband. She'll need to speak to him if this is something she decides she is curious about. I don't know him well enough.
It occurs that establishing a few poly relationships and introducing them to this couple might be a good jumping off point for them to 1) learn I'm poly and 2) see a little of how a poly relationship might work. I'll see how therapy goes; I'm not ready to be rushing into new relationships just yet.