If the shop assistant was a man who presented as a man, this wouldn't ever hit the papers. The mum and daughter would have said no, and moved on. Male shop assistants in clothes stores surely ask women of all ages if they need assistance all the time, including in lingerie or bra sections.
Especially as in many stores, assistants are told they HAVE to offer assistance to anyone who looks lost.
If a man who looked like a man who was obviously a shop assistant came up to any of us in the lingerie section and offered help, we'd just say no and (likely rightly) assume he was just doing his job, and understand that he must not have realised we were there for a bra fitting (because how could he? Ime lingerie sections have bras, pants, socks, pyjamas, night gowns, all sorts unrelated to bra fittings )
That's the transphobia and what's irrational here. People arnt all up in arms because a male employee offered assistance in the lingerie department. No one would give a flying fuck about that. They're up in arms because a trans employee offered assistance in the lingerie department .
If that young teen girl was more scared of the scenario than she would have been going to a male doctor/having a male lifeguard at the pool/any other manner of things that involve men, then that's because she's been taught to fear trans people specifically - and, as far as I know, men who are not trans ("cis") are way more dangerous to girls and women than trans people.
It's more likely that her strong reaction was because first bras can be embarrassing and nerve wracking... And pearl clutching mum has just made it worse.
Men who want to prey on women and girls don't put on a wig and dress to do it. They just bloody do it. Surely we all know that.