The ideology the Holy Father addressed and criticized in the speech referenced above, gender deconstructionist theory, has nothing whatsoever to do with the medical diagnosis and treatment of gender dysphoria (formerly called transsexualism), and the fact that Cardinal Antonelli makes the jump from pp. Benedict’s speech to transsexualism, only proves that Cardinal Antonelli has no clue about what the diagnosis entails.
The medical diagnosis of gender dysphoria, and the treatment thereof, is not based on gender deconstructionist theory. On the contrary, a transgender individual does not think that they can (or want to) change their gender. Most report having been that way from birth. The Pope effectively (and rightly!) criticized gender deconstructionism in his speech. But he did not criticize the medical phenomenon of gender dysphoria, nor the treatment thereof. If he intended to, he still did not do it - it is simply not contained in the text. Only someone ignorant on the subject will see the speech as an condemnation of transgenderism.
Seriously, American Conservatives only have two things to cling to in this debate - the sub secretum pastoral (not doctrinal) document, which states that sex reassignment does not change your gender, and Pope Benedict’s speech. As for the former, well, d’oh. No one claims that sex reassignment changes your gender. When a patient undergoes male-to-female sex reassignment, it is because she is, always was, and always will be female. The same obviously goes for female-to-male. As for the latter, the speech simply does not apply to transgenderism, since the ideology described does not fit with the opinion of most transgender individuals. To clarify, most transgender individuals would agree wholeheartedly with Pope Benedict’s speech.
So I can only conclude that the basis for the American Conservative stance in this specific case is nil. They are grasping for straws, and cannot even prove that the Church agrees with them. Which is good, since the Church would in that case violate what Vatican I has to say about the limitations of papal infallibility (the Pope can never pronounce on matters scientific, and gender is determined through science, not faith), much like She did during the heliocentrism feud.