The real flaw in the arguement comes down to money. A chick or guy, regardless of how they look, will marry and bang the person who's a millionaire regardless if that millionaire is missing a nose, has a twitching eye, is covered in zits, talks with a lisp, or is 80 years old.
There's also the view of the ugly person. While the cute person will look for another cute person, and some less-than-cute people will look for cute others, where does that leave the ugly bastard? The one that knows they're never going to get laid with a hot chick or guy? They have children too, believe it or not, and may find their significant other for reasons other than looks.
Lastly, some people, especially random bar guys, will more often than not bang a fat chick or ugly person 20 times before they end up getting tied with someone. Looks are a factor, but it ultimately comes down to laziness. They are technically banging said elephant chick because they believe 'there's less hassle and no games'. If you don't see these people now, you will when you're 30 and single. They're everywhere.
Some guys like big butts. They just can't lie.