Two weeks in, and the picture is starting to get a little clearer.
Seems F was NEVER happy with an 'exclusive' relationship with anyone else, but she was 'trying' to be for my sake. We've argued nearly every day since about why she couldn't just have been honest and spared everyone the heartache! But I think we've hammered it out enough, and its now 'understood' that exclusivity is not going to sit well with her just now, and that was the main reason for her wanting to explore doing things together.
We've visited a swingers club together, and played with some single guys, which F has enjoyed a lot, and has helped her to break down some of her initial objections to other people involved and confirmed her view that she would like to find people who want to be involved with us, together. In poly terms then, we'd be looking for a triad or couple to get to know (and in F's words 'fall into bed with') which to her mind is more natural (and perhaps 'fair')..
We do have a few couply friends, but I would be wary of trying to turn any of them into a relationship that goes further than friendship, because most of our friends know each other, and if something goes awry, it'll affect the whole group balance. So we're attempting to find swingers who are open to a little more.. We've made profiles on a website, and are starting dialogues, and we'll see where that goes..