I interpret the song as being about sexual intercourse. "Pasolini is me" and "Visconti is me" I interpret as Morrissey, or his protagonist, saying that he's gay. "I entered nothing/and nothing entered me/till you came/with the key" I interpret quite literally, i.e. nothing entered the protagonists rear and he penetrated no one else's. And then this unknown male came with the key and locked up as well his heart as his rear. "As I live and breathe/you have killed me/you have killed me/yes, I walk around - somehow/but you have killed me/you have killed me" I think can be interpret in two ways. The first, and vulgar one, in relation to the intercourse, it being a bit rough and exhausting, the protagonist feeling totally exhausted afterwards. The penetration has killed him so to say. The other way to interpret it is that the unknown male just used him and left him with a broken heart, as dead. And then the lines about forgiving him can mean, that even if he broke his heart and left him for dead at least he gave him something; pleasure, sexual liberation or whatever.
I don't know, maybe I read too much about Morrissey's sexuality into this song.