Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic draws a comparison between two situations where you know it is time to leave. The first panel shows the classic scenario of coal miners with a canary in a cage -- the "canary in the coal mine" being a well-known early warning system where a dead canary indicates dangerous gas levels, telling miners it is time to get out.

The second panel presents the analogous social version: "How you know when to leave the movement." A woman holds up a document that reads "You may only have sex with these people in these ways for these reasons." The joke equates an ideological or social movement that begins dictating the intimate details of members' sex lives to the metaphorical dead canary -- a clear warning sign that the movement has become authoritarian or cult-like and it is time to leave. The comic satirizes how some movements start with reasonable goals but gradually escalate their demands for control over members' personal lives.