*sigh* Which leaves Harry.
Harry has been great through all of this, and has made it clear that we'll be friends no matter what. But, a) he thinks I'm making a big mistake by having a serious relationship with a monogamously oriented person (his ex-wife was mono), and b) he's upset by how abrupt this all was (certainly can't blame him there). I've explained to him as well as I can why I've decided to try to make things work with Davis and why this is a really, really different thing than his situation with his mono ex-wife (she was a bitch). I've also explained to him that I will absolutely not be making any deeper commitments to Davis, such as marriage, until I'm sure that he truly can be ok and happy with a poly partner (not that I'd consider marriage any time remotely soon anyway). He's accepted that, even though he still thinks it's a bad idea.
As for the abruptness issue? Yeah, totally my fault. I apologized to Harry and told him that my mistake was clear in hindsight. I knew that Davis was very important to me, and that he'd never really gotten to a place where he could deal with the knowledge of me having other loves, and so it should have followed logically that something would eventually have to change. But I ignored that fact and so, when the crisis came, I was completely unprepared to deal with it and other people in my life got burned (thankfully, there was no one else I was particularly close to aside from Harry, so there was no one else I had to have such a tough discussion with).
We agreed that we both care about each other, and that it would be better to take some time before seeing each other again. Our friendship was very sexual, and it would probably be hard to pull back completely right away. When we do see each other again it will be in a group situation, like a get-together at my place with my roommates. I've told him that I know I can't expect for him to still be available and interested once Davis has worked out his issues, but that I very much hope he will be.
I missed Harry a lot for the first couple of days after our long email exchanges and even cried over him a little once, in Davis's arms. But it's all faded significantly now. In the end, what Harry and I had was something real, and something very fun, but it wasn't yet what you could call love, and so I was willing to set it aside for something that definitely is. I know that it was the right decision for me.