So what's your excuse.... Really funny email joke

Two teens had been lovers for a few weeks, but the boy was 
always after the girl to quit smoking. One afternoon, she lit up 
after some love making, and he said, "You really ought to quit."

She, getting tired of his nagging, said, "I really enjoy a good 
cigarette after sex."

He replied, "But they stunt your growth." She asked if he ever 
smoked, and he replied that he had never.

Smiling and lifting her gaze to his groin, she said, "So, what's 
your excuse then?"