By coincidence, I was puzzling over my own sudden lack of sexual desire just yesterday. While I am not in a relationship, my roommate recently had her 21st birthday, at which there were a number of young women who, I am told, would have enjoyed the opportunity for a casual sexual encounter.
For reasons that are unfathomable to me, I did not wish to pursue such a possibility.
My best guess is that I did not connect with the prospects on an emotional or intellectual level, and that their levels of physical attractiveness were insufficient to offset the detrimental effects of this disconnection.
However, I do not know for sure. I have filed this lack of knowledge away as a worrisome data point. Hopefully it will not become a pattern.