It's all down to how much the need for intimacy is linked to your self esteem, and if you consider your sex life to be one of the "tell-tale" signs that things aren't well in your relationship.
I've always felt that when the sex becomes less frequent or "one sided" (i.e. the other person is doing it out of obligation or guilt) then there is something rotten in the state of denmark.
However, I've never been married. It strikes me that all relationships go through this lull. Wow. It really makes me worry to be honest.
Cheers
R