I disagree that this is harmless. It's patently obvious what sex I am, and referring to me accordingly isn't a matter of continual angst or self-reflection. On that account I'm clear about three points:
I'm not holding up a flag announcing my adherence to a set of gendered stereotypes I consider harmful.
I want equal respect for my own right to self-determination. Following this practice means that is sorely lacking.
I will not constantly draw attention to my sex in a society and a workplace where I have personally experienced discrimination because I am female.
I strongly reject that these considerations make me 'transphobic'. I don't want to offer a comment on other people's right to present as they wish. I would simply like the basic courtesy of being allowed to remain silent. Yet in this climate it's inceasingly hard to obey the 'never discuss religion or politics' mantra without being viewed as an 'unsafe' person.
I'm in academia. These practices are entrenched and not at all uncommon. When put on the spot in a meeting I have answered 'that's not a practice I follow'. With a dropdown menu I would probably answer 'any, or none'.
I don't use pronouns I know would offend others. However, I've engaged in linguistic contortionism for years to avoid using sex-based pronouns. It's hard, but you do get practised at it. It kills two birds with one stone. You avoid compromising your own principles and also do not inadvertently offend anyone.