I understand that, but I fail to see how it's especially offensive women? I consider it primarily against good taste. ;)
It is sexism if you understand sexism and genderism in its strictest sense: That there is no difference between man and woman - sex is a thing imposed by society - and mentioning things like the penis, the biological difference between man and woman, constructs that change, thus is sexism.
Yeah, as crazy as it sounds.
Secondly, if a woman were to say that verbally to a man, there would be a flirtatious connotation, which would lead women to feel uncomfortable wearing a shirt saying the same and hanging around with a load of strangers.
It marks the territory as a boys club. It basically says "for brogrammers only".