Let’s start with Coileán’s proposal: when we speak of “sex” we are talking about physical morphology (and, by extension, those things that stem directly from it); and when we speak of “gender”, we’re discussing a socially shared concept, a “construct” to use the academic vernacular, and the experiences of being “of” that gender.
Do the experiences of a person of the female sex go into the sex column or the gender column?
Therein lies the problem. Within the scope of an argument about rape, if one is disgusted by a large range of male behaviors one has seen and happens to be male one’s self, does one apologize “on behalf of one’s sex”? Or are those behaviors the behaviors of a gender? Depending on whether you think those behaviors are bolted onto maleness in such a way that the behaviors will always be present if males continue to exist, or, instead, think that rape is part of a set of culturally defined personality-and-behavior stuff that makes up the gender “Man” (or “masculinity” or whatever), you could argue for either.
The transsexual-battery-hate crime thread was started by a person who apparently believes that a rigid and inflexible relationship exists between sex and the gender that is conventionally linked to it. If you are male, you are a man. If you put on certain garments and exhibit certain behaviors that are culturally designated as “Women’s”, you are "fooling men into having sex with " you; who you are is not “Woman”, and you are not a “she”; nor (for that matter) are you a “Man” who is simply choosing to dress in apparel that you like and flirt as you tend to flirt with people you find attractive and interesting, because the apparel and the flirting is “of women”, meaning it is “of females” and you, not being female, are therefore doing it wrongly, “fooling” men into having sex with you. So here’s an extreme world-view where essentially there is no gender as distinguishable from sex.
On the other hand, does gender float around ephemerally disconnected from biological sex? Given the assertions that there are more than merely two genders – e.g., instead of just “man” and “woman” we can have “straight man” and “gay man” and “straight woman” and “lesbian”, not to mention “bisexual man” and “butch” (see Stone Butch Blues) and M-to-F transsexual, even conceptual newcomers like me, “heterosexual sissy” – are there any restrictions on gender that stem from biology? Unlike Hermann Cheruscan, many of you would accept a male person declaring xy-self to be a “Woman”, not a “Man”, and saying “I was born in the wrong body”. But suppose I were to state that I am a “Woman”, not a “Man”, but that I was not born in the wrong body, this here male body is just fine with “Womanly” me? And if you do accept that as a reasonable gender identity, what, then, does it mean when Susan over there identifies as a “Woman” (whether straight or lesbian)? At this point, we’re at the opposite pole from Hermann and his ain’t-no-gender extreme – we are now essentially asserting that identity-wise there is only gender. Physiological plumbing doesn’t have a sense of identity of its own. You don’t say or think “I was harassed on my way out of the parking lot by a person who, as best as I could tell from the body parts I could see, was possessed of balls and a cock (not counting out the possibility of undescended testicles or surgical truncations, and entirely ignoring genotype)”. Quite aside from being awkward in a non-nudist world, it’s also irrelevant once you’ve decided that it means only what it is and has no implications for personality, behavior, or experience.
So you start off with what ought to be a simple distinction and still get damn little agreement on what is sex and what is gender when speaking of behavior and identity, even when people understand and agree to the definitions. And in the resulting spray of confusion, where folks hear one person say “sex” in the same context where someone else would say “gender”, you soon get folks using the words interchangeably.