I don't understand how humans being animals makes being bi curious normal? It's not an animal instinct. We are designed to reproduce, that is our base instinct and bisexuality has absolutely nothing to do with this. If anything it is the reverse and it is the development of our species that has led to it, as the pleasure over rides the rule.
Having said that, I'm not sure if men are meant to have cocks up their bottoms or fingers, but they definitely do have a g-spot up their bum and that is purely for pleasure, so I do not think wanting to have it stimulated has anything to do with bisexuality, as apposed to pleasure.
I'd like to say that I liked being played with by women and it was perfectly natural, but even if I can appreciate that a woman is attractive, she does nothing to stir my body and the idea of being touched by another woman is enough to get me running for the hills. I have no problem playing in the same room as a woman, I can even share a man with a woman, but basically I am a greedy woman who needs/wants all the attention and won't be satisified if I don't get it. *sulky sue if I don't*.