The pact you WILLINGLY make in high school that when graduation rolls around and you are still a virgin, you have sex (full-on intercourse, none of this third-base or oral crap) with your best friend of the opposite gender regardless of circumstances, sexual orientation, feelings, etc.
Please do not rape your best friend. Mutual consent is required, seduction is allowed, protection is always necessary.
Steve: Oh man I can't believe you're going to college a virgin
Greg: Naw me and Kelsey made the Sex pact. I'm bangin her tonight
*highfives are exchanged*