The church does not have theology describing transgenderism, as this is mostly within the domain of medicine and biology. Through natural law, we understand that there are two genders, and that gender is integral to an individual’s identity. Thus there can be no “change” of gender. However, determining the true gender can be complicated or impossible for some individuals.
Throughout the centuries, there have been “ambiguous” genders, where individuals had genitalia that appeared part male, part female. Until genetic testing was invented, there was no way to determine the true gender of an individual. Even with genetic testing, there are cases of people with a chimeric blend of male and female DNA in different parts of their body. Theology has simply not caught up with scientific observation yet; thus these individuals are, out of caution, barred from the priesthood, and discouraged from attempting marriage.
In the case of “transsexuals”, those who are outwardly pure male or female, but feel trapped in a body of the wrong gender, science has not yet come to fully understand the situation, thus theology cannot fully address it either. On a case by case basis, individuals may choose to undergo surgery to change the outward appearance of their gender to relieve the extreme stress they experience, but this is not recognized as and authentic change in gender. The theology is not strong enough to absolutely bar such surgery, nor accept it as a gender “reassignment”.
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Social teaching dictates that these individuals be treated with dignity and not be unjustly discriminated against.**