“Because obviously that is, by its very definition, a homophobic prejudice.”
If we are going to smear people by appealing to definitions let’s have a look:
“homophobia: harmful or unfair things a person does based on a fear or dislike of gay people or queer people”
So unless not having sex with someone is “harmful or unfair” then it is not homophobic.
Is it a prejudice? Well, yes to the extent any preference is a prejudice. If she wasn’t attracted to fat men, shy men, skinny men, overly muscular men, smokers, gregarious men, etc. But I do wonder if you were to hear a woman say she wasn’t in to really loud, out going, body-builder men who smoked you would describe that as “prejudice” or just accept it as a preference?
You refer to your own “prejudices” in this context but I wonder if you do so in everyday life, do you say “I am prejudiced against short men” when discussing your preferences? I suspect not.
Not sure why you have a problem with the very common preference of women choosing men who are straight?