Why It Is This Way
Biological
So far, everyone alive has come into this world by a perfectly natural means. Any tendency to be gay, lesbian, bisexual, or transgendered is not inherited (in the Mendelian sense,) but it is biological in nature. Our genetic code is like a blueprint, but not all the information is used in construction, and 99% of the design is the same for all human beings on the planet. Much of the design is redundant evolutionary ballast that serves as a backup, in case of catastrophic mutation or errors in the development of an organism. There is also a probability of any gene sequence being executed (during development), much like a quantum mechanical effect.
There is a tendency to think of biological processes in terms of black and white outcomes, as if the blueprint was being followed precisely. However, these processes are actually shades of gray, full of twists and turns, governed by complex rules of probability. While the process might be repeatable, the outcome (even given the same starting genetic code) is not repeatable. It is a miracle that such a non-linear developmental process can actually successfully map from the molecular level to the macro level of a living organism.
Lastly, there are hormones, specifically sex-related hormones. The two primary examples are testosterone and estrogen, though there are other related hormones, and all of these hormones are similar to the extent that our bodies can change some into others. These hormones can have profound influence on our physical appearance and our behavior. Were testosterone not a naturally-occurring hormone, it would be classified as a controlled substance, because of its pervasive power to drive our psyches in very irrational ways.
Psychological
Everyone has primal instincts, driving them to actions that, though they might seem logical, are often not quite rational. Instincts are part of our inner nature, are activated without thinking, and are nearly immune to boredom, while behavior is a choice and is subject to boredom. These instincts can cause us to use pseudo-logic to bypass all logic and do things that are inexplicable or even embarrassing. The desire to pair off in committed sexual relationships is a primal instinct. We invented marriage as a attempt to impose some control over all this madness. Marriage might be defined as a pairing of two people (who hardly know each other) in an improbable or impossible relationship full of unreasonable expectations, cultural rules, hormonal reactions, and role-playing. People might coast through all the problems for years, but eventually they will have to confront the changing nature of the relationship and deal with all the differences, conflicts, and secrets.
The more people who exhibit a certain instinct, the more it is approved, regardless how irrational it might be. Pairing and sexuality are at least tolerated, since 99+% of the population is in some way interested. Far fewer persons (5% to 10% of the population) exhibit same-sex attraction and/or gender-variance, so approval is harder to come by. Even fewer persons (about 1% of the population) exhibit severe gender identity variance, which is often disapproved as counter-cultural behavior. Yet even fewer (about 0.1% of the population) exhibit extreme gender-variance. So, it is hard for such a small segment of the population to generate enough influence to obtain their desired civil-rights. Everyone wants to fit in. If they cannot do so, depression, guilt, frustration, alienation, confliction, and suicide can result.
My Take
I am in the 0.1% of the population who have a an extreme form of gender identity variance. My personal nuances of identity are not typical, since even persons in this small segment of the population can exhibit wide variance in the nature of their gender identities.
I strive to be logical, and I dream of returning to a logical base, but this seems to be a lost cause, due to the atypical nature of my mind. I have even moved the transwoman identity into the logic portion of my mind; while this facilitates easy expression, it makes my gender-variant behavior impossible to turn off.
I follow a strict protocol of expression. I view myself as male, regardless of which gender-mode I present in. I am not the least bit interested in lingerie, but whichever gender-mode I am in, I always wear the correct clothing, including underwear. I am shy and introverted, avoid attracting attention, and always strive to be exceedingly logical. For me, being like a woman, including wearing dresses and skirts, is without any fantastic excitement. I readily admit that, for me, being like a woman is inconvenient, but that does not stop me from expressing myself as a woman. Were it not for my gender-variant nature, my inborn quest for logic would induce me to avoid social contact and all relationships. In such a case, I would neither be married nor socially involved. Remember, women are judged in terms of their relationships, while men are judged in terms of their separateness.
My situation cannot be solved the normal linear methods of dealing with problems, especially since I am always expanding my relational network. I certainly have the capacity to become a transsexual or be a transwoman full-time; however, since I am very process-oriented, I cannot transition because I do not have a valid process to do so. My situation is full of interdependent polarities, which must be managed instead of being solved. I am choosing to lead a dual-gendered life, but I freely admit that I am actually transsexually-oriented, not dual-gendered. I am not burdened by any prescribed quota or frequency, and I actually have few rules and limitations.
I seem to have the mental aptitude for dual-gendered expression, I pass well enough, and I am comfortable enough (as a transwoman) to successfully alternate my gender. I have developed a kind of logic, which might be mistaken for preoccupation, where I automatically set aside part of my time to present as a transwoman. However, I need all sorts of skills I was never taught when I was young, so I have to spend time learning about presentation, speech, communication, negotiation, relationships, and boundaries.
While primal instinct is the cause for my gender-variant identity, the presence of women is my trigger. During adolescence, I seemed to have imprinted on the image of women around me, who mostly wore dresses and skirts in public at that time. When I first began to feel this way at age eleven, the girls were all entering puberty, on their way to becoming women. I was becoming sexually interested interested in women, but I also wanted to physically emulate them in public. This was very unnerving for me; I thought women were trying to exert some dark influence on me, so I resisted both the desire to be with women and the desire to be like them. This was one reason I married so late in life and waited so long to admit my inner feelings.
Since I am always around women, the switch in my mind is firing every waking moment, and it will not stop. The presence of women destroys my composure and knocks me out of balance. When I see any nice-looking women (including my wife), I feel compelled to do something about it. Sexual expression is not enough (as it would be for most men), and I have no desire for sexual expression with any woman other than my wife. While my sex drive is decreasing, my desire for regular expression as a transwoman is increasing.
From the time I was young, I have dreamed of being able to be like a woman in public. If I do not validate this instinctive identity, my gender-variant behavior just becomes a trip. Expressing as a transwoman is not a psychological gimmick for me; it is a psychological necessity for maintaining balance in my life. Without this balance, I feel emotionally and socially inept.
My Latest Theory
I have long felt that my right-brain, left-brain connections were far different from average. I mix and match everything, and I can function just as well as a woman as a man. Perceived reality for me is obviously different than for most males. I know that many of my thoughts process are like those of men, but I feel the majority are more like those of women. I feel this compels me to feel bound to the culture of women than that of men. However, this cultural division has historically been judged in terms of culturally-defined feminine and masculine traits, rather than the actual, more subtle differences between men and women.
After watching Science of the Sexes on the Discovery Channel, I had a new idea. From the start, boys and girls have differently-wired brains; in fact, girls have a fantastic amount of interconnectivity that seems to be mostly lacking in boys. Also, it is patently impossible to turn a girl into a boy or a boy into a girl, unless the affected individual is highly transgendered. So, what if this wiring affects gender identity, and what happens if an individual gets wiring that is more like that of the opposite sex, or somewhere in between? Biology is practical, meaning that many configurations perform multiple functions.
Since individuals with totally reversed wiring would be rare, this could explain the small percentage of those who truly feel trapped in the wrong body from the moment they could talk. Most transgendered persons would be somewhere in between. This could explain what has really baffled researchers, in that most transgendered persons and even most transsexuals do not fit convenient profiles.