LatinLoven,
Lexapro is an SSRI, (an antidepressant). Some antidepressants lower libido and mine is fine (probably increased)on this med.
Her birth control is doing a number on her, I would get off it.
From a females perspective I think she is feeling that maybe she is wasting her time with you and that you may never commit to marriage. She may feel you have been on some level using her for 6 years....that she gave you all her love and youth and now she will end up with nothing. There could be some passive aggressiveness to this. I know that sounds harsh but I'm just giving you a woman's view.
If you do get married and have kids...the years with young babies you will have less sex and that's pretty much guaranteed....it's all worth it though.
I think she is having some issues with your willingness to be a lifelong partner and the birth control. I would definitely recommend counseling together. You have 6 years in this, whatever you decide you really want to make the right decision, that is a lot of time to invest.
Good luck!