Q: Why do women wear makeup and perfume?
A: Because they're ugly and they stink.
Why I think this joke is funny and not really all that offensive:
It's obviously meant to be ironic - if you take the obvious meaning (i.e. the joke-teller thinks women are ugly and smelly), you've missed the point completely.
I hear the joke as poking fun at the idea that women must wear makeup and perfume to be beautiful. It is mocking those women who apply makeup with a trowel and airbrush. Of course they need so much makeup and perfume - they are so concerned about their looks that they have become ugly on the inside, nothing but an attractively-painted shell of a person. I believe this joke is saying that true feminine beauty is not, and can never be, the product of makeovers, age-defying moisturizers, exfoliating skin creams, or anything of olay.
This joke is not about degrading but empowering women - Why do you use makeup? It it because you are ugly? OF COURSE NOT. I really believe every woman on god's green earth is a rare and gorgeous creature in her own unique way. And I like this joke because it says "You are beautiful the way you are. Makeup and perfume is for girls that are ugly and stink."
Of course, it only says that to the girls that are willing to listen.
And guys like me, who may or may not have their heads up their asses.