I was going to suggest exactly that to @RoseWatercress, @Sunset6! I think that Rose should arrange a romantic meal for them both in their favourite restaurant, and take along a written message - or send him a text while sitting opposite him - and ask him to quietly read through her note, and to let what she says in it, sink in (for just a couple of minutes). They can then start talking about how they will work out the logistics of what she is proposing. I think that her note could say something along the lines of:
" I have now realised just how important it is to you that we shoulld try having a poly relationship with another woman, but as you know, I am not at all drawn to having a bisexual relationship, but I feel as if I should give it a go at least once, just for you, and then maybe we will both love it, and can take it from there!
However, I genuinely need to enter into this sort of relationship - sharing you with someone else - very gently and cautiously, so I need you to agree to us starting a polyamorous relationship first with a handsome, intelligent and healthy bisexual man. I believe that if we did that first, for say 3 to 6 months, I could find out if I am even up to sharing you. I know that you really love me and care about my happiness, so I am sure that when you have had a few minutes to think about it, you will understand what a brilliant idea this is. Actually, I am getting quite excited about the thought of it now!"
Of course, in reality, whatever her dh's response to that is, at the end of the evening she can tell him that he will be getting a letter from her solicitor in a few days time, setting out her terms for the divorce. Unless, of course, he has had some sort of epiphany, and realises that what he wanted crossed over the line from being a personal fantasy, to him both stupidly and arrogantly, thinking that his fantasy was totally reasonable, and that Rose should go along with it! If Rose still loves him, and somehow hasn't got the 'ick' regarding him, then she would hopefully be in a very strong position to map out the only way she would consider staying in their relationship.....
(I think that it would probably involve some couples' counselling, and maybe some individual counselling for him).