Hi Alex - I'm so sorry, but I agree with Tim. If you've been together this long and she's unwilling to change (and blaming you) it's only going to get worse if you get married. If sex is important to you, then you are opening up the door to a lot of heartache in your future - heartache that's going to get worse, not better, than it is now. My husband was not very sexual while we were dating, and promised it would get better when we married, but it's only gotten worse.
Of course, if you feel you can live without sex (unlikely, since you are posting here after all) then go ahead and marry her. But don't expect things to change...EVER. Sorry.