Carbon Bonding
Explanation
The Joke
A man romantically tells his partner, "Baby, you and I are like two carbon atoms that came together to form a single bond." She finds this wonderful. In the next panel, labeled "Later," the woman tells a friend, "Wait, doesn't you have a boyfriend?" The friend replies, "Yeah, but he basically said I could get with three other people" -- referring to the fact that carbon atoms that form a single bond still have three remaining bonding sites available.
The Humor
The joke exploits the gap between the romantic intent of a chemistry metaphor and its literal chemical implications. In chemistry, a carbon atom can form four covalent bonds. If two carbon atoms share a single bond, each still has three unused bonding sites -- meaning each can bond with three more atoms. The man intended the analogy to sound devoted and intimate ("we came together"), but the woman interprets the chemistry literally and correctly: a single bond between two carbons means each partner still has room for three more connections. The metaphor inadvertently becomes a declaration of an open relationship. The humor lies in the danger of using scientific analogies in romantic contexts without thinking through all the implications.
References
Carbon has four valence electrons and can form up to four covalent bonds. A single bond between two carbon atoms (as in ethane, C2H6) leaves each carbon with three remaining bonds, typically filled by hydrogen atoms or other groups. If the man had said "double bond" or "triple bond," the woman would only have room for two or one more partner, respectively.