cucurbits
Explanation
The Joke
God is shown creating a new family of plants, telling someone (presumably Adam or an angel) that He is making "Cucurbitaceae" -- the botanical family that includes gourds, squashes, and cucumbers. God describes the family as containing "gourds, squash, cucumbers, and several hundred other botanically fascinating members, as well as plants known with modest yielding interiors." The other character sensibly suggests that God "should really not put that near the humans," implying that humans will inevitably misuse these plants. God dismisses the concern but then also warns to "stay away from the Tree of Knowledge," echoing the Genesis story.
The bonus panel shows a tag reading "This bonus comic courtesy buyers of 'Soonish.' Click for more info!" -- this was part of a promotional series Weiner ran for his book "Soonish" (co-authored with his wife Kelly Weinersmith), which explored upcoming technologies.
The Humor
The joke works on the innuendo that cucumbers and gourds have a well-known reputation for being used as... anatomical substitutes. God is presented as a naive creator who does not foresee how humans will repurpose His innocent botanical creations, while His companion immediately grasps the problem. The humor is amplified by God's oblivious enthusiasm for the scientific details of the Cucurbitaceae family, delivered in the tone of a proud botanist, while the audience and the other character can see the obvious issue. The callback to the Tree of Knowledge adds an extra layer -- God is worried about the wrong forbidden fruit.
References
Cucurbitaceae is the real botanical family name for gourds, squashes, melons, and cucumbers. The comic was part of the "Soonish" bonus comic series promoting the Weinersmith book of the same name.