In every relationship there are two positions that are relative to each other: one partner is high desire and one partner is low desire. The high desire partner usually works hard at trying to make their low desire partner into a high desire partner, but this just does not work.
We do not notice the difference early on or until a certain point in the development of our relationship, usually between 2-5 years. We then go to work to resolve it (as I said, trying to resolve it the way most people try will not work).
The important thing is to understand that this is a normal and natural thing to happen in a long term relationship. All relationships operate by the same mechanics (details may be different, but same dynamic occurs).
Your best bet is to learn how to 'differentiate'. You can learn more about this by looking up a great book called 'Intimacy and Desire' by David Schnarch.