My response to this has to be put in two distinct parts:
People are free to be upset about absolutely anything they like and I'm not going to take issue with it Its when major societal pressure is actively brought into force that I begin potentially taking issue. So they can be upset about said T-shirt all they like.
One of the details of free expression that I had to deal with in arriving at my current position is that in order for free expression to exist, that it must be possible for people to say things that I think shouldn't be said. This is one of those clashes of ideals , and we disagree on which should be the victor. Admittedly if there was some clear way of setting up a truly objective arbiter of areas where free expression is necessary for a free society then this is one of those things that should be restrained only to those arena's where its absolutely necessary (and those are few in number), I'd like most actual racial slurs in that category really, so I'm not nearly pure on this, but that's a necessity of applying any ideals to a messy reality its just that I perceive no such arbiter (such things are inevitably subject to societal whims and political efficacy) and I'd rather not have the free political expression that is fundamentally necessary to democracy potentially subject to political/social scrutiny.
As an addendum I'm not sure that your example is a perfect reflection: a) I'm not American, so I lack a great deal of the ingrained societal context that makes that so offensive on a gut level, my understanding of its offensiveness is intellectual and b) I'm not sure that female representation in STEM being genderflipped is equivalent to a raceflip which includes a significant period of time of literal slavery but I've done my best to address this ignoring these issues.