You mention you just can't have sex when you don't want to. I can understand that statement.....BUT, if your wife brought this up as an issue for her in your marriage....did you make any efforts to fulfill her needs?
I have to tell you, if you were having sex with your wife prior to marriage....and then stopped six months into it, if I were her....I'd feel like you led me on until you had me. That what my marriage suddenly turned into, wasn't what I thought it was, it wasn't what I signed up for.
I'm not judging you by any means...just giving you a woman's perspective on it. People simply don't usually get married to be celibate or virtually celibate. Very often they marry the person that they want to have that intimate connection with, exclusively. Sure, there are some situations where as the marriage goes on things do change and one person can find their libido changing. BUT if you suddenly stopped having sex with your wife, I cannot impress on you the impact that had on her ego and self-esteem.
So I am curious....why, do you think, you didn't feel like having sex with your wife?
