perfectly good culture-type question, I would have thought, though I'm not sure what it's got to do with English and I'm not sure it has an answer. Going back as far as the war, many women got an unexpected taste of freedom when they took over the jobs of men who'd gone off to fight (google Rosie the Riveter for a US example); after the war, however, they had to be encouraged to return to the 'traditionally feminine' roles of wife, mother and housekeeper, so husbands could get their jobs back. (History shows that having the streets full of unemployed ex-soldiers is dangerous.)
By the 60s and 70s, as women more routinely took jobs, and kept them after marriage and even after having children, femininity was widely seen as a snare and delusion, a form of social pressure that kept them down; and since then females have gone for equality with males in ladette behaviour, full of binge-drinking and casual sex, as you can see from some of the trollops on AB.
BUT... what all this has to do with underwear is unclear. You're lucky to get a bra that fits, let alone one that expresses your attitude to femininity. My own guess is that whether you get them from M&S or Agent Provocateur is down to whether you feel physically comfortable in them, and who's going to see them, rather than how you see femininity.
There now, that's not much help, is it?