Your girl wasted no time with her ex as you continued to mow his yard for free while they get to fuck. Afterwards you get to come in and fold his laundry.
Your girlfriend and roommate bonded when you went on your weekend fishing trip. You were reeling in an 8 inch trout, while she was taking 8 inches of cock.
Your girlfriend needed a ride but didn't have money to pitch in for gas. Good thing she's got a valuable set of skills and assets that can be used to barter.