When having doggy-style sex, and the woman refuses anal, you finish having sex with her anyways. Before you pull out, stick your finger up your own ass. You will then use this finger to wipe poop above her upper lip, making a Mexican style dirty sanchez mustache, making sure not to alert her.
Now let her venture into public with the shit mustache.
I gave that ve-och Martha a secret sanchez before she went to work. Man, will she be red faced when she finds out.