Sex change theory
The theory of "sex change" is based on the claim that a person of one gender can be born "trapped" in an opposite-gender body. The theory plays havoc with traditional ideas of masculinity and femininity. It is referred to as "transgenderism" by its liberal proponents, but can be better seen and understood as a type of mental disorder called gender confusion.
Gender-confused patients seek to undergo surgical sex change operations (also known as gender reassignment surgery, or what amounts to surgical mutilation) to change their external appearance to that of the gender with which they claim to identify. Patients often undergo psychotherapy, hormone treatment, and facial surgery as well. In the United States, a patient must undergo counseling before being approved for the surgery. Patients in the US are also required to have two letters of recommendation from certified psychiatrists, and must have lived a year acting as the gender that they claim to identify with.  Patients sometimes choose to have the surgery done abroad in places like Thailand for three reasons: 1. It is often less expensive for the patient, 2. The patient may be able to bypass some legal restrictions, 3. The surgeons in Thailand have a reputation of being particularly skilled in this area. 
It should be noted that hormone therapy and removing and adding body parts does not change the genetic makeup of an individual. On the genetic level, individuals who started as men are still men no matter what appearance they choose, and women who started as women are still women after surgery as well.
Different states have different laws regarding the status of people who have had sex change operations. Generally, their birth certificates are reissued with their new claimed legal gender. Since they are not required to divorce before surgery, this could lead to de-facto same-sex "marriage" if they stay with their spouse. The country of Brazil offers free sex change operations because they are claimed to be a constitutional "right".
J. Michael Bailey wrote a book which disagrees with the "born trapped" theory.
- In his book, he argued that some people born male who want to cross genders are driven primarily by an erotic fascination with themselves as women. 
Dr. Bailey has come under tremendous social and administrative pressure, motivated by political correctness, from liberal partisans who disagree with his scientific views based on political and personal motivation. This pressure has focused primarily upon objections to the implications of his theory, rather than objections to his empirical evidence. Rather than try to refute his empirical findings, "transgender activists worked to try to ruin Bailey professionally and personally." 
According to the Vatican,
- people who have undergone a sex-change operation cannot enter into a valid marriage, either because they would be "marrying" someone of the same sex in the eyes of the church or because their mental state casts doubt on their ability to make and uphold their "marriage" vows. 
- Christine (born George) Jorgensen: An American World War II veteran, George became Christine following a 1952 operation. This was the first widely publicized sex change surgery.
- Walter Carlos, 1967
- Jan (born James) Morris: A British World War II veteran, reporter, and father, Morris underwent sex change surgery in 1972.
- Renée Richards (born Richard Raskind): An American eye surgeon and tennis player. Richards made waves in 1976, when he entered amateur tennis competitions as a "woman".
- Chastity ("Chaz") Bono: Daughter of Sonny and Cher. Underwent surgical mutilation in 2009, and legally became a "man" when the Los Angeles County Superior Court approved her name and gender "change" on May 6, 2010.
- Larry ("Lana") Wachowski: Co-Director of The Matrix trilogy.
- Laverne Cox (birth name unknown): First transsexual actor to be nominated for a Primetime Emmy. 
- Gender confusion
- Answering parents' questions on gender confusion in children at Focus on the Family
- "The key point is that the (transsexual) surgical operation is so superficial and external that it does not change the personality. If the person was male, he remains male. If she was female, she remains female." 
- Christine Jorgensen site
- Jan Morris: A Profile
- The Lady Regrets