Preemptive sex is the act of having sex with someone before they realize they want to have sex with you. Its something that you do to someone, similar to "SUPRISE BUTTSECKS." This is a broader defintion that encompasses both anal intercourse and vaginal intercourse. This is NOT considered rape, as the person will eventually consent, just not before you have sex with them.
"Dude im totally gonna have preemptive sex with Jane tonight, she'll want it next week im sure.