Threesome
Threesome
Megan: We had a threesome last night. Cueball: How was it?
Megan: Awkward - it was with a physicist. Cueball: Why's that awkward?
Megan: They can't solve the three-body problem. Cueball: Ah, yes.

The N-Body Problem in physics refers to our inability to analytically solve sets of differential equations modelling gravitational attraction between more than two bodies. Simply put, there are exact equations for describing the movement of two bodies reacting to each other's gravitational pull, but no such solutions exist for systems of three or more bodies.
A threesome is a sexual encounter with three people. The punchline of the comic is a play on the word "bodies": a threesome involves three (human) bodies, and it is implied that the physicist's participation in the threesome was hindered due to their inability to solve for the movement of said bodies (i.e. the physicist was apparently unable to get comfortable and sexually satisfied at the same time).
According to the title text, Megan proposed that they settle for a numerical solution since a closed-form solution is unavailable. A numerical solution to predict the motion of a system would be an "open" procedural solution or simulation. On the one hand, such a solution can be more practical and less time-consuming than a closed-form solution, especially in cases (such as the three-body problem) where the latter is suspected to be impossible. On the other hand, a numerical solution is only an approximation and will tend to deviate from the exact solution over time. In the context of the comic, Megan probably was suggesting that a numerical solution would be sufficiently accurate for the duration of the threesome, but it appears that the physicist insisted that they arrive at an exact solution.
A frequent observation in random n-body encounters is that one or more bodies are ejected from the system by achieving escape velocity (and loneliness), but stable solutions are possible. A closed-form solution would allow one to predict for how long such an arrangement would remain stable. Ironically, it could be that the physicist's insistence on finding an exact solution resulted in them being excluded/"ejected" from the threesome, which arguably would be very "awkward".
Megan: We had a threesome last night. Cueball: How was it?
Megan: Awkward - it was with a physicist. Cueball: Why's that awkward?
Megan: They can't solve the three-body problem. Cueball: Ah, yes.
