To me, professionalism is oppression because personal expression one may have is repressed in support of a larger organizational goal. Now, depending on the situation, this may be necessary (like working in customer service). I would contend that it is not necessary here- which is why ESA team members are allowed to dress casually in the first place. One should not cloak all office work under the guise of professionalism. Different offices have different organizational goals they need to achieve, many of which do not require office staff to dress "professionally" to achieve and exceed goals. The ESA is one of these organizations, and it's goals were met without professional dress (clearly).
With respect to the larger issue of women being dissuaded from professional fields, there is a valid argument to be had that women may be turned away from professions that allow people to wear shirts with scantily clad and objectified women that offend them. However, in my opinion, that sentiment alone is not enough to ban them. I'm for all forms of expression being tolerated to the greatest extent possible. The recent string of fraternities (and apparently New Zealanders) wearing blackface offends me as a black person, but it does not offend me to the extent that I would ban them, or really speak out against them (in effect, banning them). I simply don't really care because they don't really affect my day to day life. I wish others would do the same, but apparently that's not going to happen anytime soon. That's why I oppose people being speaking out about this, because I wish they would stop doing that.
To put it another way. Living in Alabama and Georgia, lots of white people around here ride around with Confederate flags on their cars and have them flying outside their homes. That offends me, and doesn't make me feel welcome in their neighborhood. I don't speak out about it (even though I could) because I support their (legal) right to fly the Confederate flag and the social state where they are not ridiculed in public for flying it. On the other side of the coin, we're allowed to blast uncut rap music as loud as we want in our neighborhoods that probably offends their sensibilities. Live and let live.
