Well, DubDub, as it turns out, the last verse is indeed a parody. Upon googling I discovered that Irving Berlin wrote it, not Cole Porter. I reproduce it in white to protect those who are prurient-sensitive:

You're the top!
You're Miss Pinkham's tonic.
You're the top!
You're a high colonic.
You're the burning heat of a bridal suite in use
You're the breasts of Venus,
You're King Kong's penis,
You're self-abuse.
You're an arch from the Rome collection.
You're the starch in a groom's erection.
I'm a eunuch who has undergone an 'op',
But if, baby, I'm the bottom
You're the top.

http://ernunnos.livejournal.com/819203.html

also

http://www.time.com/time/sampler/article/0,8599,190220,00.html