As to the "Forbidden Underage Love" category...how could you forget Neil Diamond's immortal(and possibly immoral)classic "Girl, You'll Be A Woman Soon"?
Another song in the "Bodily Function" would be "Undercover Angel", by Alan O'Day...an ode to the subject of one of his wet dreams or wanking sessions, depending on how you look at it(not that you'd WANT to look at it too much).