Hey

first off, I only read the first page as a result of my attention span. so sorry if this has already been said. but, THAT said...

the emphasis of Black Francis on the "we're chained" and the sore/anguished sear of the guitar lead me to believe the aim is more at a deeper pain than just sex.

one aspect of it is that people, in a way that's more instinct and nature than simply choice, are pulled towards sex. there's the obvious tone of tiredness of the process-- whores and routine "uh"s and the continual stream of "heys" as the narrator goes for another. but of course, tug against it as much as we may, we're chained.

then it takes sex, that natural tendency of man, and goes on to say that it fuels on the next step of man, which both are met by a grunt of either pleasure or pain or boredom. the next time "we're chained" is sung, it still means the same as the first one, with us being chained by our natural instincts, but it adds the chain of our connection/obligation to the rest of mankind and the chain (from the child's perspective) of being born out of and into the "uh" all over again.

so I'd go as far as summing the point as- Man is forever bound to his instincts and to the people around him, so too bad, go ahead and keep to it.