I like to think that though Snape is too much of a gentleman to slap the piss out of Petunia, he put a curse on her anyway.

"I curse you, Petunia Evans, to marry an obese, obnoxious, middle-managing muggle, who you will have sex with exactly once to produce a sniveling little shit of a son who will mirror his father in every way, and live out your life in a little row house in Surrey, Merlin help you. And also, that your sister will still be a babe in fifteen years, while you will look like a cat whose neutering operation went horribly awry, and whose only sex appeal will be your boobs."

Thus spaketh Severus Snape, future Hero of the Boy Who Lived.