It’s seen as transphobic as to repeatedly state it without context is obviously intended as an attack on trans women.
But by the same token, it's a gynophobic attack on women to repeatedly state "some women have penises, get over it."
This isn't an issue of what we would say to individuals in our circle of acquaintances any more, it's now a political battle over whether women, the historically oldest oppressed group in the world, have the right to self-define themselves, to have their own spaces, their own political movements, to describe themselves as a distinct group from the men who have historically done the oppressing.
If "woman" ceases to mean woman, then we can't talk about sex-based violence, sex-based discrimination, sex-based oppression any more. If we roll over and allow the nonsense statement "some women have penises" to go unchallenged, we are allowing ourselves to be shat on from a great height.
Women's changing rooms - gone. Women's domestic violence shelters - gone (men carry out 90% of violent offences, against both men and women). Women's rape crisis centres - gone (men make up 98% of sex offenders). Women's right to ask for female medical professionals to carry out intimate exams - gone. Elderly, infirm women's right to ask for a female carer to wipe their arse - gone. Women's sports - gone. Women prisoners' right to be imprisoned free from the threat (and actuality) of male violence that they literally cannot escape from - gone.
We - women - did not start this fight. We were happy to accord the old-fashioned, dysphoric transsexuals the courtesy of using "she" and calling them by female names, of pretending we didn't notice they were in fact male. And in exchange, decent, old-fashioned transsexuals didn't push women's boundaries.
But the new school of transgender warriors - the Danielle Muscato "suck my dick on international women's day" variety - they are dangerous, they are intent on colonising women's spaces as an exercise in male dominance, and we cannot let them go unchallenged.