Generalizations are a pet peeve of mine -- especially when they cross the line and become stereotypes. It's said that you can't judge a book by its cover. This is true, but it works both ways. Just because the cover is attractive doesn't mean the book is any good. Likewise, just because the cover is attractive doesn't mean the book is bad.
Extrapolating from a single instance or incident is usually pretty dumb. What would the response be if someone said, "I know a woman with a great personality, but she's lousy in bed; therefore all women with personality are lousy in bed," or "I know a Christian who is a hypocrite, so all Christians must be hypocrites," or "I know a computer programmer who's an asshole, so they're all assholes." Yes, I know, in the last case at least they'd be right :)
The line of thought displayed here seems tangled up in envy, self-esteem and self-doubt. Envy is easy to understand. We'd all like to look great, or have Bill Gates' money. But when you combine envy with low self-esteem you often get this type of attitude: Many people choose to denigrate that which they want, but don't or can't have. Thus, they validate their own self-perceived shortcomings.
Of course there's another explanation. Perhaps to ease self-doubts regarding our sexual prowess we must blame our partner if things don't click. Hey, I'm OK, you're not. It's a lot easier to blame someone else than accept the possibility that we share blame.
This is nothing more than self-gratifying rationalization. It says more about who we are and our perception of ourselves than the person or group of whom we are speaking.