Well, you're damned if you do, damned if you don't. Apparently, whether you're having sex alone or with someone else, you're gonna go blind.
Two PhD students at U.C. Berkeley with a good sense of humor and a blog
found a tale in the PubMed database about a man who would go temporarily blind every time he had sex
. Every time the guy would (ahem) finish, he experienced vasoconstriction, which narrowed the walls of his blood vessels and in turn, restricted the flow to his eyes, causing him temporary blindness. Oddly enough, this happened only after sex, never after physically exerting himself in any other way.
The anonymous patient was treated with a vasodilator to widen his blood vessels, and the condition did not repeat itself. Hopefully this was before some opportunistic partner ran off with his Xbox while he was lying there in post-coital bliss / utter darkness.