They don't, really.
They just tell you that because either, 1. they're
out of your league socially, personality-wise, intellectually, or, as
TheBooBooKitty so subtly underlines,
physically, or 2. they sense you're just
settling for them until someone "normal" shows an interest, and would be inclined to refer to them as
fat, ugly, or both. If a
normal guy (i.e., someone who gives the appearance of having standards beyond body type and how many
layers of makeup they're wearing) were to approach them, there's a definite possibility that they
wouldn't have a
boyfriend.
If you make it apparent that
all you're interested in is a pretty face, you'll likely be a
bore to all women you attempt to
pick up. And, contrary to some peoples' belief, most "fat/ugly" women are
not desperate enough to go home with any
shallow loser who shows a passing interest, no matter what suspect allusions he might make to
the sexual talents of women of proportion.
Every girl on this planet does not have a boyfriend. However, it's probably safe to assume that they would all appreciate some
respect. If you're having problems meeting
single girls, try approaching women as
people, rather than as
ornaments or sexual conquests. 'Cause, hell, you're probably no
Ewan MacGregor, either, and you wouldn't want that minor detail to put you out of the game. You expect to woo her with your
wit and charm, right? Suprisingly enough, given the chance, she may well have the ability to display those same characteristics.. if you can get past her
sub-par supermodel potential.