Okay so I thought I would address the "why is this thread so long" complaint some have brought forth.
I think that the reason as to why this spawns a lot of discussion is that the shirt fits really well into two differing larger narratives, one where the shirt is a symbol of the casual sexism within STEM and one where there exist an "outrage culture" that overreach. And if proponents of either narrative neglect or choose to ignore the existence of the other narrative they can argue ad infinitum since any argument that the opponent present isn't compatible with their narrative and therefore doesn't carry any weight. Very interestingly having this singular narrative creates a need to mould real life events so that they fit that narrative which is why you can see people not accepting Matt Taylor's (in my opinion really nice and heartfelt) apology where he refers to wearing the shirt as "a mistake" since that acknowledges the other narrative. Instead Matt's voice gets dismissed in favour of "defending" him against the perceived mob of outraged people, i.e. he didn't mean what he said and he was forced to do this. As a note to that particular example:
There exist evidence that suggests he meant what he said, and there exist none for him just being forced to apologise.
The fact that the internet in general and, at the moment, the geekier side of it in particular is not very articulate and civil when it is discussing issues of gender and minorities that evokes a lot of bile which further creates more discussion albeit discussion of a distinctly lesser quality. To expand on the bit about "the geekier side of it in particular";
It has been suggested that the ongoing gamergate mess is a new front in "the culture wars" (calling it a war is absolutely ridiculous), this time centring around geek culture or to be more precise gaming. This also has a larger narrative to feed and being part of geek culture makes it likely that you're interested in the Rosetta mission. Seeing as that movement has a lot of people's blood running, when an opportunity presented itself the mess was quite ready to spill over and infect the discussion about the shirt. It is hardly surprising that several proponents of gamergate are arguing against the initial criticism to further their agenda, which yet again creates more discussion even if it doesn't have much bearing on what actually was being said.
Add all this to the fact that contentious threads usually gets more posts than more straightforward ones, it shouldn't be surprising (or considered proof of the fall of mankind) that this thread accumulated more posts than the one about the Rosetta mission. Sorry about the double post but I felt that this needed to be a separate entity.