Is he a sexist? From the background information most reasonable people agree it doesn't seem like it. Is the shirt sexist? Hell if I know. If it was a
Botticelli on the shirt would people be in uproar? Was Rumsfeld acting as a paragon of feminism when he
covered up the Spirit of Justice? To what degree does the quality of the art, posing, gender of the artist or the wearer determine if it's sexist? A significant amount of the controversy is probably attributable to the subjective nature of those criteria. If a female scientist had instead worn a shirt with a bunch of shirtless dudes, I could see some of the people complaining about it now saying it was empowering, and some of the people defending it now complaining that it proved a sexist double standard.
My impression is that it's not sexist in itself, but it's not a clear line because context matters. It comes down to where you think the line between individual expression and responsibility to society should fall. I see where the people who say it may inadvertently send the wrong message in this context are coming from. I also believe there's a way to convey that without being an asshole about it to the point that you make the dude break down in tears. That's not just for his benefit, but also for the benefit of the cause you are trying to support. Nobody likes a bully, and by being one you run the risk of making the issue about you instead of the issue itself. Sometimes I wonder how much certain people care about an issue versus how much they care about being holier than thou.
This isn't some foaming at the mouth misogynist. He's just a dude who spent the last several years helping send a tin can hurtling 4 billion miles through the inky void to land on space rock instead of studying the nuances of the causes of gender inequality. Wearing the shirt in this context is arguably a mistake, but to claim it somehow undermines everything he and the other team members accomplished (as in the Verge article) is the kind of mean-spirited sarcastic hyperbole that makes the very people you would hope to convince roll their eyes. It's counterproductive because instead of convincing people of the importance of your issue, you instead draw a line in the sand. On one side are the people who are always right about everything (you and everyone who agrees with you), and on the other are the people who are wrong (human excrement). There's no room for conversation, for human imperfection, because you've already set the terms, either people agree with you or they are trash not worth listening to. There's no room for grey--for the possibility that this is just an average dude who maybe has questionable fashion tastes. There needs to be a middle ground between the perfect angels and literally Hitler, because if the standard is perfection no one can meet it, so why should they bother trying?
I mean it's easy to forgive the guy now if you complained about his shirt because he apologized, but what if he hadn't? It's possible to disagree with someone's choices without hating them or turning them into the enemy, but when you isolate yourself into a group of like-minded people it's easy to lose perspective. That's as true of people in the "boys' club" as it is of the internet activists who might hope to change it. We could all (myself included) do with practicing a little more empathy.