First of all, there cannot actually be an alteration of one's real birth-sex. All that can possibly be accomplished is the plastic reconstruction of one's anatomical structures to assume some resemblance of the other [sex]. (Jerry Leach)The argument is that living a trans life is pointless because SRS changes the genitals but does not change their sex. This is not exclusively Christian, or even religious, but Christians are especially concerned with things around birth and conception and with things changing.
There are multiple problems with this argument. First is the false underlying assumption that all trans people, or even all transexuals, want to change their sex. Only 3% of all trans people try to change their body's sex. Of the subgroup of transexuals, changing sex is usually not the point. They tend to be more focused on changing how people perceive their sex. Transexuals did exist before SRS. Changing their sex is certainly valuable to them, but in the absence of SRS, they would live their lives basically the same.
Now, please read Trans 101: Sex & Gender. Thus, whether or not sex can change depends on what we mean by "sex." It's silly to say, "if Sex is defined by genes, then transexuality is wrong but if Sex means genitals then transexuality is moral." This implies that "sex" has some intrinsic, untouchable meaning given by someone other than simple humans. While the various concepts of sex are important, the three letters making up the word are not. No definition is right or wrong per se but we will consider if one definition is better than another in a quantifiable way. Now by what definitions of sex can sex change? Consider what sex characteristics we are capable of changing:
Now someone might say, "my definition is the right definition and yours is false." Read Semantics to understand background about this. I don't think any definition is true or false, only that the holistic definition is more useful than the others. I think the holistic definition is the most helpful, followed closely by the psychology definition.
The genes-only definition is less useful for several reasons:
Defining sex as equivalent to genes also brings up great trans proplems for those who believe everyone needs to live according to their sex. I would also ask of the person holding this view, "If a gene therapy were invented that would in fact change the karyotype of every cell in someone's body from female to male or vice versa, would the person have changed sex?" Seeing how far we've come with other sex treatment, I don't think such a treatment is impossible. To really rile them up, ask, "If that genetic change was performed upon you without your permission, would you then be that new sex and need to live according to their gender roles?" Or, a situation that actually happens, "Your daughter is 16 and hasn't had her period. The doctor discovers she has AIS and is gentically male. Do you tell her to live her life as a male and a man?" Are they consistent in doing unto themselves as they do unto others?
This definition is rather unsubstantial without the genetic component, whoever. Sex ceases to have a biological basis and instead has a historical basis. That's a problem because if there's one thing everyone agrees on about sex, it is that sex is biological. It reveals what I believe is the truth behind people believeing this and the genetic definition: their idea of sex is not tied to science at all but to the traditional idea of gender. We are wrong to think they model gender after biology - they actually model biology after gender.
Other problems include many of the same problems as the genetic definition and contains none of the advantages; it is certainly not more scientific and not simpler than genes . Additionally, I cannot think of a single instance in which any social, psychological, or medical situation in which your sex at birth is important and your present sex is not. In some states a birth certificate, which is based on birth sex, can not change gender along with other documents, but that helps only the people who want the trans person to live a harder life by having inconsistent documentation. AIS is still a problem situation here because the person's sex wasn't know at birth, only afterward. Is an AIS patient female because her phenotype was female at birth or are they male because her genes were male at birth?
I don't agree with this definition either, but it's far improved over the other two. Sex is determined by the set of reproductive organs and genitals possessed. A uterus with a vagina indicate a female; testicles with a penis indicate a male. In intersex cases, the sex is assigned by which appears closest. Klinefelter patients are male, but many cases like AIS (immature testes and vagina) aren't decided or are decided on a case by case basis. Similarly, someone who is female except infertile, has a permanent contraceptive, or has a hysterectomy is considered female and likewise for males. Because this definition is set by biologists who don't care for behavior or psychology, there's nothing explicit for trans people.
Male to female transexuals would certainly be considered still male. Female to males are ambiguous, however, because a ftm can give an egg to a female host uterus which she can use, together with sperm, resulting in successful pregnancy. If "male" is defined one as giving gametes to a female for reproduction, then the sex of a ftm is male. The ftm is potentially female as well; see Trans 101: Sex & Gender. However, if "male" requires that the gamete fertilizes the host's egg, the ftm is not male. As far as I know, this is entirely new territory for reproduction and the definition may not be thought through this far. Thus, only FtMs can be considered male and only with a subset of the original definition; by another subset definition they are considered their birth sex.
Hermaphroditic animals come into play now where they may or may not be some precedent. I'm not sure. Regardless, biologists have no hesitation to recognize that sex change does exist in other species. Sow bugs, shrimp, oysters, some frogs, and many tropical fish such as clownfish as examples of males becoming females or females becoming males. And don't forget those dinosaurs in Jurassic Park! Presumably, if a techniques were developed so that FtMs could produce sperm and MtFs contain a functioning uterus and ovaries, each would be assigned their reproductive sex.
I'm not a biologist, but this definition strikes me as very useful in studying biology where reproduction is of prime importance. There is one comparison that makes me suspicious of how helpful this definition is outside biology. An infertile female, an infertile intersex "female" and a transwoman all cannot reproduce because they do not have functioning ovaries and/or a uterus. In a definition concerned about reproduction, is it strange that three individuals with the same reproductive function have three different sex assignements (female, male and un-assigned)? Theoretically it's useful, but not in a practical sense. And what good is a definition for something so important as sex if its practicality is inconsistent?