I'm sure the T-shirt was just a joke, but I hope you can see it is a tad offensive?

Hardly offensive, given that the t-short is a perfectly valid expression of a central tenet of Gnosticism. To quote from the site:

"Some of them say that the serpent was Sophia herself; for this reason it was opposed to the maker of Adam and gave knowledge to men, and therefore is called the wisest of all [Gen. 3:1]. And the position of our intestines through which food is taken in, and their shape, shows that the hidden Mother in the shape of a serpent is a substance within us."

(St. Iranaeus of Lyon, Against Heresies: The Sethian-Ophites, pub ~ A.D. 180, from Willis Barnstone's The Other Bible, p.664, pub. A.D.1984)