Lesbian/Bi ?

I don't think I understand. Well, I do. It is people of the same sex who love eachother. But is it the exact same as a straight relationship? I just could never see myself loving a girl in the same way I love my boy. I feel the need to have someone bigger and stronger then me, that is masculine and I know will protect me. Is it a learned behavior, or are people born with their sexual orientation? Wouldn't a lesbian couple feel like their is something missing? I suppose not. But I can't seem to rap my head around it, and I want to understand.
I don't mean any disrespect.