SixStringer, sounds like he hit the nail on the head!!
This sounds like the power of suggestion gone awry. They say that the biggest sex organ is the one located between the ears, and our thoughts hold a lot of power. For now, I would try to mix things up a bit by doing oral sex, "69" position, handjob, anal, and sex in other places such as the living room or den, for examples, and see what happens. In time, this situation should improve, as there really is no logical basis for it.