Of course sex differences exist. I’ve never denied it. I don’t take estrogen just because it makes my skin look amazing.
My arguments are:
1) perceived sex differences are far more malleable than cis people with little interaction or familiarity with trans people seem to understand. After a year on estrogen I’m two inches shorter, the orientation of my hips have changed such that they appear wider. I experience symptoms consistent with a period once a month. My breasts are larger than some cis women I know (and still growing), and have mammary glands and milk ducts just as a cis woman has. I get treated as a *female* all the time. Many of us do, even when meeting and interacting with doctors for whom these sex-based differences actually matter and would, presumably, be best able to identify them.
2) sex as a definition is circular. A female human is someone with female traits, and we know they’re female traits because female humans have them. This is a manifestation of oppositional sexism - the notion that male and female represent absolute, distinct, mutually exclusive categories. This notion hurts women generally as it enforces an ideal that all women automatically conform to this specific circular definition, when the reality is the opposite: no woman conforms to this ideal absolutely.
3) because of oppositional sexism, a sort of backwards logic is applied to trans people, and trans women especially. Since we are born as men, it is presumed that we are, under it all, men, with no overlap into the other sex. Any relationship to female characteristics are presumed therefore to be false, artificial, or superficial. And, crucially, any deviation from the ideal female form is presumed to stem from our fundamental maleness, rather than being chalked up to an inherent diversity in sex-characteristics and appearance. A trans woman and a cis woman could have precisely the same body proportions, and the cis woman would be seen as a woman and we would be seen as a man due to our obviously mannish shoulders and obviously mannish hips.
My viewpoint is: having a preference for plumbing is not transphobic. Expressing the preference is not transphobic when and where appropriate. Intrinsically tying a specific plumbing configuration to a specific gender or sexual orientation *is* transphobic, as it is a denial that we are the gender we say we are, which is my yardstick definition of transphobia.
Put more succinctly: