It still doesn't make sense. I'm guessing you are trying to make a joke out of the tired, frankly sexist, cliche of Serena being a bit manly, so you don't find her sexually attractive, but then you'd also have to be suggesting that Letterman looks like a woman that you do find attractive.
For sarcasm to work, there has to be a joke in there. Saying a sporty woman looks like a man is mean, not funny.
.....anyway, got to go as I'm up at 4.30....going to New York tomorrow.....see ya.