A lot of "Impossible Soul" seems to deal with Sufjan's experience in and out of the closet; along those lines, this piece seems to me like an address to a woman that he dated, apologizing for the pain he may have caused her. "Boy, we can do much more together" may be her suggestion that they stay together, with "Girl, I want nothing less than pleasure" being the narrator's denial to accept a relationship that would not please him (eg, a relationship with a woman)
A lot of "Impossible Soul" seems to deal with Sufjan's experience in and out of the closet; along those lines, this piece seems to me like an address to a woman that he dated, apologizing for the pain he may have caused her. "Boy, we can do much more together" may be her suggestion that they stay together, with "Girl, I want nothing less than pleasure" being the narrator's denial to accept a relationship that would not please him (eg, a relationship with a woman)