This works best when your girlfriend refuses to have sex with you due your sexually transmitted disease.
Tell your girlfriend you understand what she wants and plan an encounter in two weeks. Until that day do not wipe your bum after bowel movements, thereby creating a thick crust of dingleberries and residue in and around your crack.
On the night of the date, turn the heating way up, to around 100 degrees.
When she comes over engage her making sure you are on top. At this point the sweat which will inevitably accumulate will pick up the residue on your butt and seap down into her vagina and anus, flowing down from the north inflaming everything in its path.