As soon as I read this sentence: "If you hold very traditional, mainstream views about relationships and especially marriage, then you probably won’t like what I’m about to say" I knew what was coming
Congratulations on making the decision to take this next step! I look forward to reading what you have to say about it, and I hope it works out well for you and Erin. I completely understand what you say about physical and emotional intimacy--I feel the same way, though I don't think polyamory is for me.
I'm curious, you say if you'd made this decision long ago you probably wouldn't have gotten married. What about the legal benefits of marriage? I'm not just talking about taxes, but also medical responsibilities, inheritance, and things like that. Are those irrelevant to you?