I have found that too, the thoughts are just not there. Too many other things to think about regarding running the house, kids, work etc. I don't consciously think about these things NOT to think about sex either.
Sex is not a need it is a want. Sure it's a need for the survival of the species but for the individual it is not. You won't die without it. Some of us just don't have that physical desire to be bunny rabbits. For the sake of my marriage I'm gonna try and find some though!