"Bad compliment" joke

Two high-school buddies were attending the senior prom.
"Suzy wants to go out to my car. She's really hot," one boy said. "I'm really nervous. I know I'll goof up!"
"Take it easy," his friend assured him. "All you gotta do is compliment her. Chicks love to be complimented. You'll have her in the palm of your hand."
About a half-hour later the young man came back, rubbing a black eye.
"Shit, man! What happened to you?!" his buddy asked.
"I took your advice."
"Didn't you compliment her?"
"Sure I did. We got in my car and started kissing. I told her that for such full lips, hers sure tasted sweet. She liked that. After a while I started feeling her tits, and I told her that for such large breasts they sure were firm. She like that too."
"It sounds like you were doing great," his friend said.
"Well," the other answered, "that's when everything went wrong. I got her dress up and her panties off, and I tried to think of another compliment."
"What did you say?"
"For such a large crack, it doesn't stink much."