I think you're wanting it both ways.
If a man doesn't become a woman by putting on a dress etc (I agree) then dresses and bras are not clothes for women and girls, they are clothes for anyone who wants them.
So no I don't want bra adverts that say that bras are fir girls. It's not M&S's job to police who buys their clothes. Anyone who wants to buy them can do so. Of any sex or gender. Let clothes be clothes.
And "thing" isn't dehumanising. "Bright Young Things" is a well known phrase describing the cohort of youth about to burst forth into adulthood any minute now.