She thinks he may be unfaithful, so she asks her sister to help her with something.
As the boyfriend pulls up to the house and enters to find a trail of roses leading up to a closed door. He opens the door to find his girlfriends sister, laying on the bed.
“So, do you fancy it? You know you want to”, she asks.
The boyfriend spins round immediately and heads back down stairs towards the front door only to find his girlfriend beaming, “I knew you would never do it!” She runs over to hug him.
The moral of the story? Always keep your condoms in your car.