I have never heard the song with the image in my mind of him having sex with someone.
I always thought it was about being addicted to something that does'nt imperative has to be a drug (like LSD and so on). Maybe it's more about a habit that he has and actually hates, but he is unable to let go of the behaviour he's used to. That leads to some kind of depression, and while he's trying to fight the need to do -whatever it is- again, he watches himself fall. Into the addiction.
Hm... Just random thoughts. I need to hear it again now and force myself to NOT see him having sex with a woman. *cough*