you are trying to find a biology vs. social division in this, and that just won't happen. there's a reason why that whole nature vs. nurture argument has been tossed out, we are too far ingrained and intermixed that we cannot completely say "this is purely (this)." basically, these "preconceived notions" have been given to you even before you were aware of it. in fact, there's probably many preconceived notions still floating around in you that you haven't been forced to come face-to-face with.
just because you could not cognate with the appropriate adult definitions conceptions of gender and sex doesn't mean you weren't already being manipulated by their influence by society (or nature, btw). infancy and childhood doesn't exist in a bubble, and your awareness of them is inconsequential for them to leave an effect on you. that's where you are making the mistake. in fact, the best way to manipulate someone is by their unconscious "consent" -- make it seem normal, natural, and as they grow older, their own formulated idea. since you apparently had no "conflicts" with the biases being instilled you were often blissfully unaware, and all the more easily controlled by the conditioning. at most, any minor gender bending exhibited might have been noted, commented on, received and critiqued by you, and then possibly ignored. but even though you did not think of it as an adult, naturally, it will have left a mark on you -- you just cannot see it yet because you weren't breaking enough of the world's rules.
but think of this: all of modern western society, which was strictly defined in gender and only recently becoming so open about variance, has condoned, if not mandated cruelty and abuse to those who openly transcend sexual and gender norms. biology itself, with its day-to-day reinforcement of sexual difference (try to take a piss and then forget that there's a sexual difference in our species, i dare you

), goes about its course with its determined structure. both nature and nurture exhibit strong reinforcement of definition that sex=gender (edit: as dimly understood in our species) -- and you have young children, many thinking they are completely alone in the world because no one is there to offer them hope and acceptance of their difference, still, against the entire world, insist: "no. you are wrong. you are all wrong. the me inside, my spirit, my ego, my identity, whatever, ME, i know who I AM, and all of you, every one of you, is wrong. your entire culture, everyone who claims to love this shell of me, even nature down to my very own body, you are all wrong -- the real me is inside!"
it is a profound display of self-identity over all, just about a miracle of the human spirit. to seemingly defy both nature and nurture, and insist, against all the forces that comprise your world and upbringing, that you are real, normal, and not a mistake. that everything else is in err in judging you without your input. it is a level of personal awareness most people never achieve in their lifetimes -- because they aren't cornered into it.
so many have been given twice the gift of acceptance through conformity, yet barely realize it. one set is the conformity to society, the other is the conformity to biology, and it is the biological gift we so easily gloss over. so what if a few of you understand what it's like to gender bend a bit as a child, albeit unconsciously. when you can speak that truth in flesh that you cannot recognize as yours, then speak to the transsexual. then you will finally know that you are not your body, a mere pawn of flesh to the propagation of nature, just as you may know that you are not a mere pawn of society, a functionary enslaved to society's norms.
and that, perhaps, is the scariest and most confusing thing to people about transsexuals -- the truth that you are not your body. that you are you, somewhere unknown where decision making and identity is made, and nothing, no power on this earth, is going to steal your sense of self for obedience and comfort. that's profound, and frightening; it speaks to a power of defiance and resilience that's humbling.