My vote goes to... nurture.
For me it was my aunt. One could argue that if it wasn't her, then there would have been some other image that influenced me, but of course we'll never know
My aunt always wore sleeveless shirts so she must have been at least semi-proud of her arms. By no means were they the arms of a fbb as we know them today, but they were nice and thick and she had some definition in her tri's.
From the age of 5 or 6, maybe younger, I was consumed by her strength and muscles... way before my brain was really able to process it... and way before puberty hit me.
She has shaped my sexual tastes for the rest of my life... which... I'm sure, if she knew, she would be shocked about and/or totally unable to comprehend!