Everyone discussing this needs to see Stuart Murdoch telling NME (it's on youtube) literally that the song was written while the band were in London with Arab Strap, another band, and he was really unwell and out of it so he spent a lot of time roaming around and being solitary (on the bus from Camden etc, which passed a prison so that's in the song too). He didn't know at the time what an arab strap actually was, so the sexual stuff wasn't the point of the song at all. It's about his experience in London, which is pretty much exactly the way the song describes - posh and filthy and lonely and crowded, all at once.
Everyone discussing this needs to see Stuart Murdoch telling NME (it's on youtube) literally that the song was written while the band were in London with Arab Strap, another band, and he was really unwell and out of it so he spent a lot of time roaming around and being solitary (on the bus from Camden etc, which passed a prison so that's in the song too). He didn't know at the time what an arab strap actually was, so the sexual stuff wasn't the point of the song at all. It's about his experience in London, which is pretty much exactly the way the song describes - posh and filthy and lonely and crowded, all at once.