The drive to reproduce is one of the strongest human instincts. Along with pleasure it is at the root of our sex drive, regardless of how that drive is expressed. It is the hidden facet of family life, and it builds human society; through sexuality and reproduction, descendants are born and ancestors are created. Religion has often been the force that dictates and enforces the rules of who has sex with whom, and thus who reproduces with whom. Many religious factions condemn gay and lesbian sexual activity as being "unnatural" because it does not include the direct potential for reproduction (which is believed to be the purpose of sexuality in the first place). Physical procreation is a narrow definition of how people influence future generations; our actions and our impact on the world shape the adults that today's children will someday become. There are ways to influence the future in positive ways, and a transgender person can pass an inheritance forward in non-genetic ways.
In the case of transsexual individuals, hormone therapy and/or surgery render them sterile. Some of these people have already become parents before transitioning and making the choice to transition can cause a person to lose their family; their place in the line of descent is erased and they become a dead end. Others, particularly younger people, may have made the decision not to have children, perhaps wanting to avoid the complications of being a transsexual parent. Regardless of their choice, they may still feel the need to pass something of themselves into the future.
I never wanted children. My body always told me it was infertile, though I have never had this medically verified. I found the physical process of reproduction nauseating and rejoiced in the fact that my lesbian sexuality would never result in unintentional pregnancy. I did not see myself raising a family or being with a partner who wanted to. When Hela brought me to the realization of my transgender nature, I found a certain harmony in this childless life, but it wasn't to remain so. Because no emotional stone could be left unturned, I have had the uncomfortable pleasure of understanding the procreative drive from both a female and male perspective. I know things I feel I have no right knowing, and some of the darkest moments of my life have come from wanting things I can't have. I am an infertile woman and a sterile man, and sometimes this causes me a lot of distress.
In my research into transgender Mysteries, I came across some tantalizing information about the role transgender people play in the lives of children, and how children figure into the lives of these people. In many Native American cultures, it was the job of the transgender tribal member to mentor youth into adulthood; young people might be sent to learn skills that would prepare them for the future while living apart from their family for a time. In this way, transgender people had a direct influence on the future of their tribe; they were a formative influence on the adult that children would become, much in the way the child's parents were.
Transgender people in traditional cultures were often perceived as having special value because they could perform the tasks of a woman or a man; they were essentially two people. Since their predilections might not compel them to become parents in the physical sense, they would have had free time available that would have otherwise been spent rearing children. This time gave them the opportunity to perfect art and crafting skills, and for this reason there is a mythic element of third gender people being the bringers of culture. In the Navajo creation myth, White Shell Girl and Turquoise Boy invented pottery, and taught people how to farm.  In cultures where the roles of shaman and priest were filled by transgender people, skills such as astronomy, divination, healing and medicine formed the foundation of later arts and sciences. Some writers have gone so far as to say that "social life might never have advanced beyond [. . .] primitive phases" except for the contributions of transgender people. 
There are some practices that mimic physical reproduction, and it could be said that these are a form of third gender reproduction, or an expression of this desire. In Mojave culture, an alyha (male-to-female) may imitate menstruation by scratching herself between the legs with a stick until blood was drawn. When she and her husband wanted to become pregnant, this menstruation would cease, and when the time came for delivery, the wife would drink a bean preparation which would induce violent "labor pains." The defecation that followed would be considered a stillbirth, which would be mourned and buried by the parents.  Siberian shamans of both sexes were said to give birth to animal spirits; to the Sakha, this was an important component of a shaman's training.  The deferred sexuality of trans people may carry a special potency because it has been offered up to the deities; the gods then use the person as a vehicle to deliver blessing to those who ask for it Some cultures believe that transgender people bring a certain fortune to a baby and they are called on to bless the birth of a child; in India the hijras perform this service in the name of their goddess.
Typically we have to look to traditional cultures in order to find these clues; there is nothing closer to home. One of the most frustrating aspects of being third gender in a culture that only recognizes a binary is that I don't know where I fit. I don't see myself in other people, and my nature is not recognized as having any value. Thus, the challenge has been to create my own value. There must be cultural ground not covered by women or men; I must have a reason, or else I wouldn't have been placed here. No one will give these reasons to me, so I have to justify my own importance. How can I contribute to society? How can I make a difference?
My masculine procreative drive compels me to share, to pass information along that it may be carried into the future. I want to shape something and guide it. To that end, I see my writing as a way to express this. I can give abstract ideas a physical form with my palette of words; writing is a textured, nearly synesthetic experience that lets me feel I'm giving something concrete to the world. In this way, I can change the world and help shape the future.
My feminine procreative drive is expressed artistically, usually through spinning and knitting. I carefully choose my raw material, weighing merino and cotton, silk and rayon. What will this creation be, and how will it touch another's life? A knit item (or spun yarn) is created according to a pattern, just like DNA; knitting telescopes that pattern, using basic components to create an infinite variety of results. It grows, and changes according to the will of the knitter (or perhaps the will of the item being made); sometimes mistakes are made and there is an unexpected surprise at the end. Yarn has the same spiral shape as the double-helix DNA molecule; it is infinite, twirling over and over again, tumbling through time. It holds possibility, and has continuity with the past; yarn is made the same now as it has been since the beginning of the craft some 20,000 years ago. With its continuous spiral, yarn is a model for the creation and emergence of all things.
My third gender nature seems to have its own way of influencing the future. Once, a great SHE grabbed me and used me to pass along a blessing of fertility to a young woman who desperately wanted to have a child someday. The goddess who owns me and the god who I married both hold my sexuality; this part of me has been entirely offered to the divine so perhaps the blessing could pass through me cleanly from Her hands. I will certainly be called on to pass these blessings to other people as they come to me.
The real question is: do these things help me cope with the Niagara Falls of emotion that makes up the desire to reproduce? Sometimes. I may spin myself into a stupor, the twirling spindle only winding me tighter into the knowledge that nothing can or will grow from me. Anger at never being a father can chase my words away and leave only hot, frustrated tears. I may feel the blessing of fertility pass through me but I know it can never be mine. Both sides of my nature scream at me for weeks or months at a time, making my body into a battleground; every hour I'm reminded of what I have and don't have, and what I can't have. It is a physically painful experience. The activities I channel my frustration into don't necessarily "fix" things. As often as not, it's just a constructive way to distract myself and pass the time until I feel better. I hope that, with time, whatever alarm clock is ringing with turn itself off.
In spite of the emotional turmoil, I believe that there is some value to this experience. It compels me to act, and keeps me connected to the fact that I am not isolated. No one is a solitary entity. I am connected to others like me, who may be suffering in the same way. I am connected to the past, and I am connected to the future. The work I do now is the genetic code I pass along. Doing this, I can become an ancestor.
 Hermaphrodeities (Raven Kaldera)
 Intermediate Types among Primitive Folk (Edward Carpenter)
 Gender Tree: Mythology and Demonology - ("Sandra," after G. Devereux "Institutionalized Homosexuality of the Mohave Indians", 1937)
 The Woman in the Shaman's Body (Barbara Tedlock, Ph.D.)