sex pact

(idiomatic) an agreement about having sex with particular frequency, or in a particular way regardless of circumstances

Example: 2007, Marc Rangel, The Splendid Prize
  "She called it “sex without fear” or “sex without hang—ups.” This selfless sex pact had the added advantage that it could be arranged between any two individuals who were so inclined, whether they chose to marry or not, or whether they were otherwise compatible or not."