Seligman gives acedia as the main understudied sexual disorder:

The most mysterious and painful and costly sexual problem is "acedia," which means the waning of sexual attraction and passion between married couples as they age. Acedia is is both a physical torpor and the torpor of the soul. It is a sexual indifference that comes from familiarity. Acedia is a sexual disorder since it meets the defining criterion of a disorder, namely, it grossly imparis sexual, affectionate relations between two people who used to have them!

page 173 of What You Can Change and What You Can't by Seligman See also this summary