If a man acts as your designated driver, you owe him an equivalent favor.
Also, if your driver has managed to attract a woman and stands a reasonable chance of taking her home, he is entitled to place a request for you to find alternative transportation.
If the cab fare if more than your bar tab for the evening, he is expected but not obligated to pay half of it. This clause doesn’t apply if the man actually takes the woman home.
This law doesn’t apply if the man expects to hook up with his girlfriend, and is still obligated to get you home.