Three years ago I received an email from my then-17-year-old child. It began:

Dear Dad: You wanted to know the true reason why I hate to stand up straight and why I wear a breast binder.

I had been upset because she'd been hunched over, so we'd had her tested for scoliosis. She never wanted to go swimming, never would wear form-fitting clothes. I couldn't understand it: She simply disliked her body. So many things had happened since middle school, when we found our once-bubbly and talkative daughter sitting behind a closed door, cutting herself. Over time her depression lifted -- at least, it was managed -- but it felt like so much was still underground.

The email went on:

I am transgendered. I have known, that I was different gender-wise since fourth grade. I vividly remember being put in a group of boys for a project and feeling an enormous sense of relief and comfort because I was with people who were the same gender as me.

The email (which had come from the next bedroom) was a shock, although I had heard my wife say to our child, "You need to tell Dad too."

My GLBTQ friends call me Sebastian. This has been my name for three years -- when I first came out in 9th grade. The reason why I don't stand up straight is because it emphasizes the fact that I have breasts, which I hate. Even when I am wearing the breast binder, which drastically reduces the visibility of my breasts, I still have a substantial bump on my chest.

The email was the first time I met Sebastian, my son. That day I wrote Seb back and told him that I love him and that it would take a while for me to absorb and understand. Those were true words, because the process has been one of the most difficult I've ever experienced. Retraining myself to say "Sebastian" instead of "Anna" was relatively simple. More difficult was the pronoun therapy. Two years ago, on the way to dropping him off at college, I was being reprimanded every time I slipped. "He! He!" he yelled. It appears that pronouns are more deeply seated in the brain than first names.

Seb lived as male in his freshman year at college. (We would Skype, and I asked in wonder, "You mean everyone really believes you are a guy?") Then, last summer, Sebastian made his name change official and started taking testosterone. Last month, after dread and nail-biting anticipation on my part, he had top surgery to remove his breasts.

As a society, we have just begun to talk about what it means to be transgender, and I, like most men of my generation, knew almost nothing. If there is anything that I've learned, it's that the subject is deeply complex. I think I understand something fundamental, but I really don't. The problem stems partly from a lack of imagination. Although I am straight, I can easily imagine what it would be like to be gay or bisexual, but it is difficult for me to know what it would feel like to realize I was born into the wrong body.

After Seb came out to us, he told his aunts, my parents, and, reluctantly, his religious grandmother. Fortunately, everyone in the family expressed unconditional love and has moved to accept the truth. Over the past year my wife and I have exhaled enough to begin telling friends and neighbors (many of whom have known Sebbie his whole life). How do you honestly answer the question "How's your daughter doing in college?" when you are standing at the pharmacy? Sometimes I choose to answer, "Really well, thanks" -- avoiding pronouns -- and sometimes it's a half-hour conversation ending with a big hug.

As I walk through polite society, I continually consider what is safe to talk about. Name change and gender are givens, but the real story is trickier, because it delves into sexuality. See, Seb's first real love is another trans man. They met online about two years ago, and they have been privately traveling down the same road. The first time the two of them came to stay with us, they slept together in Seb's bedroom. I found myself glancing at the closed door and grappling with it like a cartoon character: "Two guys? Two girls?" It was hard for me, as a father, to apply the prewritten scripts. Fathers are not supposed to talk with their daughters about sex, and with sons the conversation goes something like, "Just don't get her pregnant." With Seb and his partner I was at a loss.

Maybe the real reason that transgender issues are tougher to talk about than gay and lesbian ones is that it is almost impossible to separate the idea of gender from what we have between our legs. When my 9-year-old nephew was told that Seb was going to live as a man, he angrily blurted out, "But he doesn't have a penis, does he?!" The real story of being transgender is far more than changing an "F" to an "M," changing one's name, and starting to take hormones. It's about rewriting everything that society takes for granted about what it means to be male or female -- ideas that could never be discussed in polite conversation.

When people ask me about how I feel about having my daughter become my son, I simply say, "It's becoming easier." Three years ago I would sit in the car and cry for the idea of the adult daughter I would never have. The thought of surgery filled me with revulsion and fear. The concepts were new, and I found myself wondering if my child was somehow just making it all up. But I learned that most transgender youth are in pain: Some 41 percent attempt suicide, compared with less than 2 percent of the general population. They feel like they have no way of escaping the prison of the body they were born with. Rejected by family, many have no safe place to go. I reminded myself of these facts whenever I wondered about Seb.

All I care about is having a happy and healthy child, so when Seb had top surgery last month, I stood in the recovery room relieved and deeply proud of my son's bravery. Now, after six years of tightly binding his breasts, Seb is able to walk tall, looking ahead with his shoulders back.

CORRECTION: An earlier version of this post erroneously stated that Sebastian and his partner met more than four years ago. They met about two years ago. The post has been updated accordingly.