Now, I don't know if this is true or not, but in one of my pregnancy books it says the following: "For men, however, desire may slow over the course of your pregnancy. Men who live with pregnant partners produce less testosterone and more of other hormones such as cortisol and prolactin. While this may make your man more relaxed and nurturing, it may also inhibit his sex drive."
Like I said, I don't know if this is true, but maybe this could be why?