I think looking at dicks, especially big ones, can be an intense experience for anyone, and it's hard to sort out whatever feelings accompany the experience might mean.
A more insightful question for diagnosing someone's orientation would be, to my mind, how they feel about kissing people of either gender. If a guy gets excited looking at a big cock, but thinks the idea of kissing another man is disgusting, how gay can he be?
A simple answer, to my mind...if you happen to be wanking off to pictures of big cocks, or have the desire to do so, then that seems pretty gay to me. That said, who cares?
If, on the other hand, you're just here to compare and get off on having the sight of your cock enjoyed by other people, then you're not gay; just male.
I think if people were allowed to just 'be', there'd be a lot less hang-up about sex and sexuality.