I tend to believe that the eyes tell us what to love .. but the heart tells us
who to love. (If that makes sense).
Coming from someone who has had relationships "all over the map" to exclusively b/f relationships, and is currently in a long term relationship with someone who does not identify as butch (or femme for that matter) - I think it really all comes down to how a person flips that switch for you.
She would often ask me what attracted me to Her, since She knew I was primarily attracted to butches - but She was someone I could not resist. She pulled on my heartstrings, and I fell in love. She made it go boom. Something I had no control over.
I never really factored in physical appearance as a pre-requisite - HOWEVER - there have been very few instances in my life where I have dated femmes. (It really depended on how strongly I felt about them).
Sometimes, you think you know what you want - but your heart knows best.
I don't think it's shallow if you know what attracts you. But I do also have a tendency to believe in keeping an open mind and listening from "within". But, that's just me.
Love, to me, is just that unconditional.