Maybe he's getting bored with the ussual routine. Or perhaps the demanding of sex from you. I've noticed a few times in relationships a woman consistently demanding sex = drop in guy's libido.
It's posible though like alter ego said, he finds it more physically satisfying or has some sort of fetish.
In any case, before confronting your guy think about what you can do to help and what changes can you make in your own routines in order to make the situation better.
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~