Thanks for your replies.
He's known her for about 6 months. I'd only heard of her in passing, and never by name, until very recently.
DH & I had a long conversation last night.
I told him that this was *our* marriage, and any decisions about it would be taken & discussed together, and not in the presence of, or any deference to, a third party. Sure, I'm on here asking for advice/trying to get my head round it, and fine if he'd spoken to someone who is polyamorous - but preferably after he'd spoken to me about his feelings, especially not confiding in a woman who'd be interested in having a relationship with him - 'cart before the horse' comes to mind there, and it's natural that my defences would go up, as to me all I heard is 'this woman and me want an intimate relationship, deal with it'. Unless he'd discussed the possibility of being polyamorous before, then to me it sounds like he's found someone & wants to legitimise cheating.
They'd exchanged several emails, and he showed them to me (OK so I did push him when he told me that they had, as I will not tolerate some relationship decisions being talked about, when I'm the one in the relationship & not her).
To start with she was a supportive friend, someone who he confided in when he was intially very, very down. Whilst I appreciate her support, and can understand why he was drawn in, it upset me that he confided in her things he didn't with me. That in itself feels like a betrayal - sure he may have not wanted to upset me, and she was an objective outsider, but she was a *woman*, and that upset me, opening up - emotionally - to another woman, which in this case was always going to be fraught with danger. If I'd confided in another man at that time, that would have been just as wrong.
Fact is I needed him at that time, and he needed me, and instead he betrayed me by confiding & building intimacy, discussing things of such a personal nature, and of course she encouraged it, and not once did she tell him to talk to me (which is what you should do in bereavement - talk to each other, not to 'friends'). Instead she seemed to revel in being his confidante.
She then started flattering him, talking about how much she admited him, how the fact that he & I didn't talk, was proof that one person is not enough.
She spoke disparagingly about our marriage, picking fault, reminding him of arguments we'd had that he'd told her about. Very subtly planting seeds in his mind about 'us'.
Then she brought up the idea of being in love with more than one person at a time, and confessed that she'd fallen in love with him and told him that he'd fallen in love with her, although he was scared to admit it.
To be fair DH actually told her he loved me, and wanted only me, but still she (very subtly), kept on about loving him, about him being confused, but how we all are when we fall in love with someone else.
Eventually he did confess that he 'thought' he loved her.
I've told him that I don't want him to speak to her anymore. He's not happy, but has agreed, or else I'll move out with our toddler.
I'm really not sure about polyamory. I guess I'll be flamed here, but no, I don't think I will ever accept it. I have great respect for you guys, but I know it's not for me.
If DH decides he truly is polyamorous, then we will have to end our marriage.
I have asked him to think long and hard about it. Will he be happy to be with just me for the rest of our lives? I'm not stupid, I know that situations can develop, that we can't know the future, that we can only promise to try to do what we feel today; I know there are no guarantees.
But I'd also rather just move on with my life, and DH move on with his now, if he feels that polyamory is something he wants to explore, than to live with resentment.
I don't know what else to do really