Story about a male (I think 1st person) who, no matter what happens, keeps accidentally having sex with people. Random twists of fate - teacher falling off a ladder, sister fixing a ceiling fan I think, etc. Ring any bells?
I think I've read something like this, it could be "Be careful what you wish for" which involves the protagonist ending up in sexual situations any time he is errect. This includes similar to what you mentioned but the sister falls through the ceiling from her room above straight onto his appendage as he's in the middle of masturbating.
