While from a purely genotypical and phenotypical perspective it is technically accurate to describe transsexual individuals as being male/female according to birth sex, it turns out that what defines ones gender as either male or female is rather more complex than either genetics OR genitalia.
What 20 years of feminism could not teach me, I learned when I loved someone who lived with this on a daily basis. In time I discovered that what I thought I knew about gender equality was littered with prejudice and propaganda propagated by all factions of the gender wars.
Interestingly, transpeople have often been invisible observers in the gender wars. For those who do not appear obviously transgendered (they "Pass") a world is opened up where they become privy to the secret intimate world that is guarded by each individual gender grouping.
For me, accepting transsexual women as women is about widening the parameters of what it means to be a man or a woman. To permit anyone who wants to claim that gender construct to do so if they wish - much like allowing modern feminists to be Volvo-driving soccer moms (if that's what floats their neo-feminist boats). Its about letting go of policing gender. Its about conceding that despite our desperate attempt to claim that gender is purely a societal construct, science has shown us this is NOT the case but gender IS more multifaceted than previously believed.
It really sucks for activists when their arguments dissolve in a puff of scientific smoke, but I am a scientist first, and an activist second, so my brain didn't hurt so much the day after I woke up from this particular hangover.
I've reached a place in my life where I don't fight for us to wipe clean the slate of gender constructs anymore. I fight for the right for anyone to claim whatever gender construct they choose for themselves - because to me that is true equality. That is honouring the bell-curve. That is accepting that we are both a product of nature AND nurture.
Accepting that some men have vaginas and some women have penises is really of no consequence to anyone who genuinely wants to fuck the patriarchy and free women from eons of oppression. Becoming comfortable with the reality that we can't know the content of a persons mind or underwear by looking at them is likewise a good way to advance the cause of equality. We really need to get over the compulsive need to know what whether they have the right equipment down south BEFORE we buy a person a drink. The answer is that that's just one more thing about you'll need to learn the polite way: By getting to know them.
Trans people generally fight really hard for gender equality and to destroy sexism and patriarchal power - because transmen really hate the patriarchy for
telling them that being a man isn't an option, and transwomen really hate sexism because society treats them like even bigger crap than cis women.
They hate this stuff JUST LIKE WE DO.