I don't think it's fluid because I don't believe it exists. I think the physical sex characteristics of a body are irrelevant to the qualities, behaviors, attractions, etc., of the person living inside it.
I don't believe there is a such thing as a masculine or feminine quality (except as pertains to the body itself). Strong, weak, stoic, emotional, callous, sensitive, aggressive, submissive, repressed, expressive -- all of these are HUMAN qualities. And I find it extremely offensive when someone stereotypes them as masculine or feminine. The most commonly stereotyped quality, in my opinion, is compassion. (sometimes called 'sensitivity' or 'being pussy') The ability to feel someone else's feelings and understand their experience through that. It is stereotyped as a 'feminine' quality to the point where a person who is supposedly very wise and enlightened said that "the female is the source of genuine human compassion." I find that so. fucking. sickening! So men can't have compassion? what are they, monsters who care about no one else, doomed to selfishness forever because they made the mistake of being born into a male body? Or perhaps they are beggars, who can only come about compassion by being given it by a female. And what does this say about being female? that we are to be the source of all humans, while men give nothing? ARGH. Showing emotion is a part of compassion. Crying is often an expression of compassion for yourself or someone else, and this behavior is stereotyped as feminine. I could go on and on about every one of the qualities that is commonly stereotyped as being somehow related to genitalia. And then again, about behaviors/dress. With the exception of bras, there is no real reason for any difference in clothing due to sex. And then again, about attractions. Hello, it is not genitals that are attracted to genitals, it is a person who is attracted to a person.
This is why I am bisexual/queer. People generally come in two sexes, with the rare variation, and I am attracted to the spirit within a person. I find female, male, intersexed, and transsexual people equally attractive in the same way that I find slim and thick people equally attractive. Beauty is variety.
ETA: for a more structured explanation, read the userinfo of abolishgender. I agree with it completely.
ETA #2: in this post, by 'gender' I mean social/cultural categories, (stereotyped qualities, behaviors, dress, attractions), not physical sex characteristics (genitals, reproductive organs, hormones).