Because you’re working at the level of a graduate student, I would encourage you to develop a thesis statement that allows for the application of one or more critical theories and for at least some historical contextualization. For me, the 1953 play Picnic (and the 1955 film adaptation) captures the emerging conflicts of the new generation coming of age in the 1950s who are bumping up against traditions that they find constraining in innumerable ways. Many of these traditions have to do with definitions of love and sexuality. Many people think of the 1960s as the decade of the youth counterculture, and that characterization may be accurate in a general sense, but the 1950s are clearly marked by discontent by a sizeable population who did not wish to follow the post-WWII model of the domesticated housewife and the man in the gray flannel suit. Consider the Beat writers, the Mattachine Society and Daughters of Bilitis (early gay/lesbian rights groups), the writings of Betty Friedan, the 1950s obsessions with juvenile delinquency and the corrupting influences of comic books, and so on. (I strongly recommend that you read David Halberstam's The Fifties.) I would suggest that you focus on Hal’s intrusion into the small town, the eroticism that he brings with him that challenges the established order, particular in relation to Madge, whose future has been largely planned out for her up to the point of Hal’s arrival.
Here’s an idea for a first draft of a thesis statement: “W. Inge’s play Picnic is far more than the tale of a how a single stranger can throw an entire small town into confusion. It’s also a creative exploration of the forces of social discontent that would transform American society in the coming decades.”