My girlfriend tried to make me have sex on the hood of her Honda Civic. But if I'm going to have sex, it's going to be on my own Accord.
Honda Sex Agreement
Explanation
This is a pun using the names of two Honda car models, 'Civic' and 'Accord'. The humor comes from the fact that 'accord' also means agreement or harmony, and the person is making a joke about only having sex if it's on his own terms, and on his car.