Nothing "makes you" this or that. You have an inborn sexuality, which has perhaps not yet manifested itself. You say you have been attracted to women, and you say you are aroused by the idea of sex with this particular man. (The fascination with big penises might not be strictly sexual, so it's best just to set that aside for the moment.)
If you have not actually HAD sex with either men or women, then you haven't even started to discover what your inborn sexuality might be. You have to actually HAVE sex all the way through orgasm, and find it gratifying (which you likely won't, with every single partner) before you can begin to build an idea of what you like. Simple fantasies or masturbation images won't do. Normally, this discovery process takes years. For some, it takes decades.
So don't worry about declaring a major. You can audit multiple partners, for years to come. Eventually, it'll be clear - and you will never have to decide what to "call" yourself. That's pointless. People pursue what intrigues them, NOT what their Sexual Identity Card tells them they're supposed to want. You don't need a name for it. You don't even have to understand it. Mother Nature has already sorted all this out for you. It's automatic. Forget labeling yourself.