Your sex drives might be out of sync and if you demand sex from him then this may be a ressistance on his part. Having sex when demanded (espeically in a monotonous sex life relationship) is one of the most furstrating experiences and I wouldn't blame someone for declining.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~