In traditional Christian theology, Pre-Lapsarian refers to man's innocent state before the Fall. This was the state in which Adam and Eve existed prior to their defiance of God by eating of the Tree of Knowledge.
In some respects, one could argue that Joe and Missie-May are also innocent—initially, at any rate—in that they don't seem to understand how the real world actually operates. Missie-May, in particular, is spectacularly naive, believing that the serial womanizer Slemmons with give her his gold if she sleeps with him.
Joe, for his part, expresses a desire to be just like Slemmons, with his boundless confidence and swagger. He seems not to realize that one can't just put on a different personality like that, and that, in any case, Slemmons is hardly someone whom one should look up to as a role model.
Although Joe and Missie-May's relationship experiences a lapse due to Missie-May's infidelity, they still remain committed to each other. In that sense, they are like Adam and Eve, who left the Garden of Eden together in shame to face whatever the hostile world outside had to throw at them. No longer quite so naive about the ways of the world, Joe and Missie-May's increased self-awareness allows them to begin the process of rebuilding their marriage.