You guys are wrong, its when a man and a woman engage in anal sex, then the man takes his penis and rubs it on the girls upper lip leaving a moustache.

after having anal sex, the man pulls out his penis and wipes it across his partners lip, forming an authentic looking mexican moustache.

NO, a Dirty Sanchez is performed when a male fingers a female's ass and wipes the reminants on the female's upper lip. What you referred to below, when a male inserts his penis in a female's ass then transfers the fecal matter from his penis to the female's upper lip is the Dirty Rodruigez, the first cousin of the Dirty Sanchez. Don't feel bad though, they are commonly confused.

It does not matter how you do it. It's a Fecal Mustache.

The term 'dirty sanchez' originated in the porn industry, particularly films involving ass-to-mouth (atm) action. It describes the (originally unintentional, i.e. blooper) act of leaving a shit moustache on a partner's face by taking your cock out of her ass and putting it in her mouth without wiping it off first. The result is that as the dick is thrust between the lips, shit is scraped off and comes to rest on her upper lip in a moustache configuration.

Anyone who thinks it involves the deliberate wiping of shit with fingers on a partner's face is a know-nothing fucktard.

During anal sex, you pull out and rub your cock across the top lip of your partner. thus giving her a mustache.

While having sex, the man puts his finger in the woman's asshole. He pulls his finger out an wipes it on her upper lip giving her a shit moustache.