The way “1960s” costumes seem to work in this movie is that the menswear is relatively accurate, because for men, 1960s style is all about harking back to an era were Men Were Men. Back in those days, Don Draper and James Bond could order sixteen martinis and a new pocket square from room service, and not have anyone bat an eyelash. However, 1960s womenswear just doesn’t have the same connotations, and most of the womenswear in XMFC was altered to make it look more appealing to modern standards of beauty. Most noticeably, the bra shapes. It’s absolutely plausible to include a scene where a bunch of Playboy Bunny types are entertaining gross old dudes in a members-only club, but the girls would all be wearing bullet bras, would have ~trendy ’60s hairstyles (WHERE WERE THE BEEHIVES??), and would probably be about 15 lbs heavier than the modern-day models they hired as extras in that scene.
The way I see it, the filmmakers went out of their way to create a “best of both worlds” scenario: 1960s sexism allowing them to include a scene where a whole bunch of women strip down to their underwear for male entertainment, but 21st century standards of beauty so the (male) audience can appreciate it on the same level. — The costumes of X-Men: First Class, Part 1: Womenswear.