Good
Explanation
This comic is a post-coital bedroom scene where one partner asks the classic question "Be honest -- was it good for you?" The other partner responds: "Well, to be clear, I don't take a prescriptivist approach to sex."
The humor lies in applying linguistic theory to an intimate situation. In linguistics, the "prescriptivist vs. descriptivist" debate is about whether language should follow fixed rules about what is "correct" (prescriptivism) or whether language should be studied and accepted as people actually use it (descriptivism). By saying he doesn't take a prescriptivist approach to sex, the man is essentially dodging the question -- refusing to judge the experience by any fixed standard of what sex "should" be, instead accepting whatever happened as valid by definition.
The caption "That linguistics degree just keeps coming in handy" adds another layer of humor, as linguistics degrees are often mocked for being impractical, yet here it's being used as a tool for diplomatic deflection in an awkward personal moment. The joke also implies the sex was not great, since he's using academic jargon to avoid saying so directly.