—Your Box Opener,
Granted, people are not supposed to snoop in private diaries, but you did, and what you've learned you can't unlearn. Now your best hope is to be direct—about everything. Tell her you read the unsent letter. Tell her you need clarification about obviously unresolved feelings. Tell her—for her sake as well as yours—she needs to deal with how she feels about the man who's gone and the one who's present. Then listen ... and you will know how to proceed.
I read these letters where one partner says, oh, we don't have sex, we haven't had sex, we may not be having sex, etc. I think some of these are so far-fetched (the guy who marries the girl and then says it's OK that she NEVER WANTS TO HAVE SEX comes to mind) that they are hoaxes. I mean, fake stupid letters just to be funny, you know? I think they are beneath you to even respond to on Slate's Web site. Sure, some people may have serious problems that make them not want to engage in sex. But what's the excuse for the partners? No, I really think they're fakes.
My dear, you've obviously never been on the receiving end of advice column mail. Prudie could certainly be fooled a time or two, but the number of people who write about not getting any (or not giving any) would twirl your turban. The partners on the receiving end—figuratively, obviously, in some cases—have multiple reasons for not bailing. Two would be 1) hope and 2) love.