Derek is his own worst enemy, and not just on Halloween but pretty much all the time he fucks up any possibility of getting with a girl. I'm surprised he's been with as many canonical girls as he has. Anyway, my point is that it doesnt matter if he wanted to fuck Becky or not, but that he's insulting her primarily because she's a huge bitch. He could have just as easily said she has loose floppy pussy lips, or he could literally see she was riddled with STDs, or she was slowly leaking cum out of her ass, or anything like that.
Becky would be his.... uhh let's see, girl from camp, Ashley, virgin girl from his story at breakfast, Wendy, granny... maybe ranked 3rd or 4th on his list.