This seems very confused. How about...
To make it clear that a man doesn't become a woman by putting on a dress etc (I agree) then dresses and bras need to be marketed as clothes for women and girls, to reinforce they are not clothes for anyone who wants them.
So I do want bra adverts that are clear, whether explicitly or implicitly, that children's bras are for girls. It's M&S's job to respect customers' needs, not to police who a woman or girl can be. Anyone who wants to buy anything can do so. Of any sex or gender. But not at the expense of girls' names and needs. Let girls be girls.
I didn't actually see much issue with the slogan myself, but I also understand those who do. M&S's apologetic response does imply an ulterior motive to de-sex bras & erase girls' physical reality.
I see women's anger at this as not dissimilar to an increased wariness about the sex of people using female toilets. Previously, "fearless young things" would have been a barely-acknowledged indication of progress - a reminder that there's less need than ever before to focus on sex differences one way or the other, as they're understood, respected, & largely irrelevant. And previously, a 6-foot woman with a crew cut in the loo would have been a wholly unnoticed indication of progress, a reminder, if you even noticed her, that there's less tendency than ever before to focus on gendered appearance one way or the other: it's understood, respected, & largely irrelevant.*
This was progress indeed. But now? Women have been hurt, and an injury makes you flinch at things you wouldn't have before. It's a pre-emptive protective mechanism - it's become so much harder to see things like this as innocent in a society that's constantly rubbing salt into our metaphorical wounds.
*This is emphatically not to add fuel to the popular trope that GNC women are being kicked out of toilets. Within half a second, it's clear who's who. But even that half-second of extra caution simply wasn't on our radar just 10 years ago. Trans activists caused it, just as clearly as trans activists are now those most likely to highlight and condemn it.
The ironies of all this are infuriating sometimes.