Hi Virginia
I know the expression.
It's something shrinks say when they want to prove their hypothesis.
Then they say: when it looks like a duck, it sounds like a duck, it walks like a duck and it smells like a duck...it must be a duck....so it's something to say that your theoretical assumption is the right one.
Maybe it's the same thing for women, when it looks like a woman, it sounds like a woman etc.
And of course you're excluded from some boy talk, for guys it's easier to talk about girls, without girls, because then they can tell how good they are with girls. If there is a girl nearby is only one phrase from the girl enough to destroy their image for a very long time.
So, for me it means that you're a girl in their eyes.
hugs
annette