There is:
Genetic Gender -- Chromosomal inheritance
Physical Gender - The physical aspects of your birth
Brain Gender - functional structure of the brain, especially in the Amygdala, along gender lines, and hormonally affected gray and white matter mixes and distributions in the brain.
Brain Sex- Orientation -- who you want to go to bed with
Gender Identity -- Our subjective gender, sexual Self-Map, how we feel ourselves to be. Male, Female, Androgyne, whatever.

Set these things in a matrix, and you can see there are a lot more than three, or four, or five "sexes".

Dr. Carl Bushong (http//www.doctorbushong.com/pubs/what_is_gender.asp) suggests on the basis of his studies, that our brains develop gender in at least three independent dimensions, which he calls Brain Gender, or how the brain is wired along gender lines. Brain Sex is how we perceive sex relationship and goals along male-female sets, and Gender Identity, how we perceive ourselves, Male, Female, or other. Not only are these dimensions independent from one another, they are independent of the physical identity as well. Thus a person in a male body can have a male brain sex, but have a female Brain Gender. Such a person would look, act and feel male, but have a female's sensitivity to emotions, words, and sensations, and this is frequently so among writers, artists and others. If you construct and view the matrix, you can see that the possible combinations and degrees of variance truly could number in the thousands. In fact, with regard to gender, we can be in a category of one -- ourselves.

Culture confuses the issue. Each of us operate in the world with a set of models of that world tucked into our brains. The information forming the models is derived from what we learn and are taught by the elders of our generation, and what we learn from our peers. So, in our culture which we lovingly say is scientific, we love labels, so we can put lots of labels on things. In our brains labels are needed to be able to think about a thing and to communicate to others about it, so these are shared within a culture. So we have "heterosexual males and females with totally male or female brains and bodies that likely form maybe a few percent of our species, and then a lot of others that we pigeon hole on the basis of our perceptions of them, and these based on our very own models of the world. We can have normal, gay, lesbian, bisexual, Androgyne, Assexual, Intersexed, some of whom have gender dysphoria over an assigned gender, transsexual, transgendered or CDers, Fetishistic Transvestites, and many other terms, many prejudically uncomplementary, and all not totally correct due to the limitations of language and our mental models. What I might think of as a Transsexual, for example, you might think of as a tranny whore, dependent upon your own mental model of the world. Other cultures have other models of the world, and these are really no more correct than any other in terms of types of sex.

So you are an author. Make of it as you will for the world you are writing about, since you are writing a scifi story, you can label things any way you want and conceive of them any way you want, and be correct.

CaroL

Gender: Male

Sexual Orientation: To cover me, the scale would need to be diamond-shaped, with the middle skewing up or down towards either bi or asexuality. I'd be in the dead center of it all right now, used to be strictly asexual though, but now I'm beginning to become slightly bi-curious.

Gender Identity: Male, but really really close to the middle.

Sexual Identity: I've never actually participated in sexual activity, but from the few fantasies I've been beginning to have since my hormonal balance has been changing, and from the way I behave in social situations where I could either take charge or let others run things, I'd say I'm extremely submissive.

Aesthetic: Middling, wind up presenting as slightly more masculine for defensive purposes, but if I had my druthers, I'd lean more towards feminine. I do, in private.

Social Conduct: femme. very femme. I tend not to act out at all in social situations because I know I can't even come close to pretending to be butch enough to pass as a "normal" hetero male.

Sexual Activity: If I were interested at all, definitely monogamous... But as previously covered, I'm mostly asexual.

---

This has started changing recently with my herbally induced feminizing. Especially on the sexual orientation and activity. I think the identity as a submissive was always there, but I would never have realized it before... I'm not entirely sure I'm comfortable with the realization that I'd rather be the one getting skewered, when I don't really feel like I should be all female. I accept it, but I'm never going to act on it. Can't really... I want to get it where I don't have the right equipment to accept it, and don't want it on the other end. I also don't feel like I should have the right equipment... Ah, the complexities of it all.

And really, once I put my answers to all the categories down, the only thing keeping me from being a full transgendered is that I really do still think of myself as mostly male, just with faulty wiring.