Well unfortunately there are far too many women out there who perpetuate the myth that it's "normal" for there not to be much sex in a marriage...I'd love to just SMACK those women!!!
Ok, so she gets into it, but doesn't think about it. Have you asked her to talk to her MD or OB about this? "IF" she truly doesn't think about it, or has such small urges for it that it's easy for her to ignore, I would suggest a complete physical to begin with....a complete physical w/bloodwork. Her Dr. would need to know that she has either no libido to speak of or a very low one...so he/she knows what to look for.
OR....would she be open to seeing a MC w/you? What I've learned throughout this last year of working on this issue with my LDH is it is all too often a communication/misunderstanding issue. Your W is likely to truly just not "get it", she probably doesn't really understand what a problem this is for you...because it isn't a problem for her. A good therapist can help both of you learn to understand each other and each others needs. They can help each of you to not take things so personally, or help each of you to see how you aren't meeting the others needs...even if it's unintentionally. I've been amazed at how much my eyes have been opened....and I thought the problem was all on my H's side of the fence LOL.
Have your read the bood SSM, that is advertised on here? If not, pick it up and read it...it's a quick read and I truly do feel you'll find things in it that you will completely relate to. It may also help you understand where she's coming from.
Do you think that she would be open to reading something like this herself? Not everyone is.