Whew. Now I can follow in Mo's footsteps and not sound like I am being a rag.
I thought her form of "initiation" was complete crap.
If I were you, that would be my numero uno thing to tackle--her inability to initiate in any way except to phrase it as a "gift" that she is giving you and that you should be appreciative of. What a load and no wonder you felt no desire for her--frumpy nightgown notwithstanding. No flaming here! I think that if it was truly meant to be a gift, she would have found a way to deliver it in prettier wrapping. Meaning, ditch the stupid phrasing and come to bed nekkid, for goodness sakes.
One suggestion I have is to perhaps give her some ideas of HOW you would like her to say that she is willing to ML. I know that my H is absolutely clueless on how to initiate, or what to say. I literally have to tell him what words I would like to hear (and keep it in the realm of something he would actually be able to say). His brain just does not function that way. It may not be politically Schnarched to say that, but I do think that people have limits and PM is not a recipe for perfection--it is a way to create passion in a marriage, not give each partner their dream mate, kwim? So maybe she would benefit from the knowledge of how you would like her to say it...something along the lines of "What about ML tonight?" does not compromise her integrity (she is not saying, Oooh I want you so bad Big Daddy) but it is a clear initiation that is free from the passive aggressive sh*t that she has a tendency to dish out. And, Dave, I DO realize that she has not one iota of a bad intention when she says this stuff--I can recognize that the sista just does not know how to disclose herself in that way. Intimacy and making herself vulnerable feels very scary and uneasy for her (especially in light of the fact that she is supposed to make herself feel that way FOR YOU, she (thinks she) is not getting much out of it).
So that is my only piece of advice...teach her how to say it in a way that is not offensive to you. Repeat the words until they stick in her brain and she can successfully ditch the sarcasm that she so heavily relies on to protect herself.