For what possible reason are you asking that question of anyone?
https://en.m.wikipedia.org/wiki/Jessica_Yaniv
> In 2018, Yaniv filed discrimination complaints with the British Columbia Human Rights Tribunal against multiple waxing salons alleging that they refused to provide genital waxing to her because she is transgender.[15][16] Yaniv's case was the first major case of alleged transgender discrimination in retail in Canada.[17] Yaniv was seeking as much as $15,000 in damages from each beautician.[18] In their defence, estheticians said they lacked training on waxing male genitalia and they were not comfortable doing so for personal or religious reasons.[19] They further argued that being transgender was not the issue for them, rather having male genitalia was.[20] Yaniv rejected the claim that special training in waxing male genitalia was necessary,[21] and during the hearings equated the denial of the service to neo-Nazism.[22][16] Respondents were typically working from home, were non-white,[23] and were immigrants[24] who did not speak English. Two of the businesses were forced to shut down due to the complaints.[25]
Just one example..
> In October 2019, the Tribunal ruled against Yaniv and ordered her to pay $6,000 in restitution... The ruling was critical of Yaniv... stating that she "targeted small businesses, manufactured the conditions for a human rights complaint, and then leveraged that complaint to pursue a financial settlement from parties who were unsophisticated and unlikely to mount a proper defence."
This is a person who deliberately does all sorts of weird shit for the sole purpose of using her trans identity to sue people when they react negatively to it.
For example, she apparently called her local fire department "dozens of times" for "help getting out of the bath" (when in reality she needed no assistance) and "subjected Fire Department staff to "inappropriate and lewd conduct"".
But besides that, it doesn't answer the question you were responding to. Asking someone's birth sex does not tell you what genitalia they have. It doesn't even really tell you what genitalia they had at birth. Since those salon owners say they specifically objected to the woman's current genitalia rather than the woman's status as transgender, birth sex is the wrong question to ask.
To be clear, the beauticians explicitly didn't care (per their claims) what the birth sex of the customer was, so I'm not sure how that would be a relevant question here? Per their claims, they'd have had the same issue with a trans-man who'd had bottom surgery.
Then you’d probably want to ask “What kind of genitalia do you currently possess”. Sex assigned at birth is not a reliable indicator of that, for reasons very similar to why current gender identity isn't.
You might want to rethink that phrasing, because it’s a very reliable indicator. That’s the entire problem. <1% of the population have genitalia that doesn’t match their sex assigned at birth.
Asking someone “what genitalia they currently posses” is more offensive because it sound like you’re talking about an accessory that people swap out on whim. “Which will you be carrying with you today? The penis or the vagina?”
> You might want to rethink that phrasing, because it’s a very reliable indicator
In the context where current gender identity is an insufficiently reliable indicator, which is the context of the supposed need to ask the question, no, it is not.
If you need to probe beyond current gender identity because your personal sensitivities about genitalia can tolerate no error, then you need also to bypass indirect proxies entirely and ask the question you are actually concerned about regarding genitalia.