Latest update to this, in case anyone is wondering:
He's been through kind of a sea-change in the last week. He had been defensive, trying to justify what he did, and still keeping secrets. I didn't know what, I just knew they were there. He lied to me - a small, stupid lie, about nothing important. And because it was such a small, stupid thing that wasn't at all important, the fact that he chose to lie about it instead of telling the truth made it that much more heinous. I told him it was done, and I'd be contacting a lawyer. We really are married - got the license and everything - and we own real estate together, so this can't be done in a snit, it takes lawyers and court and divorce decrees. I guess that broke through. He called me in tears, and told me all the stuff he's been keeping a secret. He's stopped trying to justify what he's done and just admitted it was wrong and a complete betrayal. He's been fully open and honest with me. He invited me to install monitoring software on his phone, so I know where he's at, where he's been, who he's called and called him, what e-mails he's sent and received, and what text messages he's sent and received. He's opened up in his SAA meetings and started sharing. He invited me into one of his therapist appointments, and he wants to do full disclosure - tell me everything that's happened and how long it's been going on. I guess that's an important step for him, to fully face what he's done, and it'll help keep my imagination from kicking into overdrive every time he doesn't answer his phone.
If he can keep this up, I can see a time when we're a couple again. We're still sleeping in separate rooms and I don't know when that will change, assuming we keep working on this together. We went upstairs on Saturday to watch something that had been Tivo'd on the bedroom television and I couldn't even sit on the bed. I tried and couldn't.
Sometimes this seems like a mountain that's just too high to climb, but then I look at him and see the same man who I have loved and who has loved me and supported me and believed in me for the last 20 years, and I don't want to find out what my life would be like without him. I will, if he doesn't resolve this and give me assurance it's over and done with, but I don't want to. I'd be just fine without him, but I'm great with him. I've kind of gotten used to being great.