It doesn't take a genius to notice the intentional similarity of a well-preened set of female lips and a vagina.

It's odd to me how women can not consciously be aware of the signal they're sending when applying lipstick. Namely, "gentlemen - your penis goes here!" but buy me a nice handbag first.

Elsewhere on the internet, an Answers.com submission offers this:

The first recorded use of lipstick was a henna-based concoction used by Egyptian prostitutes to advertise their willingness to perform oral sex. It has also been claimed that the use of lip color made the lips on a woman's face subconsciously resemble those of the vulva.

This all seems so remarkably obvious. Women adorn their lips with hues of wet pink and red to constantly remind men of the hidden prize that awaits them below, should they be willing enough to earn it.