Well the only thing I can think of is that basically it "means" that when you watch porn, apparently only women masterbating or lesbians turn you on.
Is it normal? What do you mean by "normal" in what context? I don't think there is any research out there to support what is normal in taste in porn.
Here is a better question: "does it matter?" If you enjoy it go with it. Whats the reason you are worried about it being "normal"? Did someone say something to you? Are you unsure of your own sexuality?
You say you never fantasized about being with another woman BUT you are watching it & getting aroused and porn is fantasy. Maybe you are bi-curious? That could be a possibility and you don't want to admit it for whatever reasons.
Dunno, I'm not in your head.
The reason you are worried about whether this is normal or not would probably shed more light on the subject because really all you wrote was "why do I only like lesbian porn?"