From the very beginning, a haze of darkness hangs over the play. This darkness is the long war that has been raging on between Athens and Sparta, and that has recently taken a turn for the worse (from Athens's perspective). It is in response to this urgent crisis that Lysistrata organizes the sex-strike. Somebody's got to do something.
Pressure of Darkness
As the sex-strike goes on, the Powers That Be do their darndest to oppose it. They don't want to see any change in the existing power structure in Athens, even if it means dragging on the bloody war with Sparta. The Chorus of Old Men try to smoke out the Chorus of Old Women as they hold the Acropolis, and the Magistrate says a bunch of gross, misogynistic stuff. A couple of women try to defect from Lysistrata's army of no-nooky, but Lysistrata catches them. Myrrhine's husband begs Myrrhine for sexytimes.
Everything Comes to Light
After Cinesias has been sexually led on by Myrrhine, he meets a Herald from Sparta who is suffering from the same painful, and obvious, state of sexual arousal. The Herald reports that the rest of Sparta is, um, hard up as well.
Cinesias and the Herald decide that there's nothing else to do but make peace. In no time, the Athenians and Spartan delegates have convened and hashed out a peace treaty. The darkness of war has faded—and the men have all realized that they would rather make the beast with two backs than continue the war. Sound decision, dudefolk.