I'm positive that every girl has thought about her father's/brother's erections at some time?
A shrink once told me that every daughter compares the cocks she's exposed throughout her life to either the actual image of her daddy's cock if she's seen it, or the imaginary image of his cock if she hasn't. She said something like no man can ever live up to the thought she has about a penis because her daddy's is always going to be the biggest and most impressionable.
Just theoretical I know but there's probably some truth there.