Hi Hobnobfanatic, my dh's exg used to drive me mad with jealousy, but now I can accept her link with my dh is important to them both. She sometimes emails and they occasionally meet. I trust him now like I didn't before.
The situation changed because I finally agreed to go and meet her and her dh at request of exg and my dh (bear with me here!). This was after months of her phoning and requesting to meet him, all through my pregnancy with ds (now 9 months). She always managed to ring when something significant was happening (eg scan dates, due date! etc - very wierd) - and me being hormonal was distraut as I felt she was competing for his attention (I think she was really, but not intentionally.
Anyway, eventually we all went off for a picnic (me, dh, ds exg and her nonplussed dh) that exg arranged. This was the day after ds's 6 week check, so he was tiny and I was not at my best physically (bad hair, big tum, sleep-deprived etc). It was all very nice, in the park, spring flowers, lovely food etc, but rather strained.
When we got home, he thanked me for going and I gave him a massive rocket because I felt he had never once considered my feelings but had insisted on maintaining ties with her, as his right. I thought they'd both been selfish and idealistic. Amazingly, he was quite chastened and said he understood and was sorry.
They are more like exs now, rather than starcrossed lovers fantasising about what might have been, and I think it was necessary for us all to meet in order for them to notice that they weren't being quite fair.
So I would say it's good to do something social together, so you know her and she's not just an idea to you. And if you're a traditional type like me, tell him you want to get married - because then it's clear who he has chosen.
I think if you both want monogamy, you have to both agree it, preferably in front of an audience