"Someone may see" joke

A boy takes his girlfriend home after going out together.
When they reach the front door, he leans against the wall with one hand and says, "Sweetie, why don't you give me a blowjob?"
"What? You're crazy!"
"Don't worry, it will be quick, no problem."
"No!! Someone may see -- a relative, a neighbor..."
"At this time of the night? No one will show up..."
"I've already said No, and NO!"
"Honey, it's just a small blowie... I know you'd like it, too..."
"No! I've said NO!"
"My love... Don't be like that..."
At this moment, the girlfriend's younger sister shows up at the door in her nightgown with her hair totally in disorder. Rubbing her eyes, she says, "Dad says either you have to blow him, I have to blow him, or he will come down and give the guy a blowjob himself, but for God's sake, tell your boyfriend to take his hand off the intercom!"