It's hard to get to the bottom of a play like Pelléas and Mélisande, primarily because it was designed by the author to be true on many different levels of interpretation. And the plot is so familiar to most people that it's not exploring new possibilities about the unknown, it's actually just rehearsing ancient moral principles. It's not condemning necessarily, unless interpreted that way, but it does show that breaching a contract like the covenant of marriage has consequences that are serious and dangerous, so even without moralism, it shows something wise and practical. For all of these reasons, the story can be said to be an archetypal representation of betrayal and the cycle of rebirth. In a more specific way, the story is about understanding what it feels like to be cheated on by someone that you genuinely love.
When the daughter is born to Mélisande, it isn't clear what her fate will be. For that time in history, being born premature is almost surely deadly, but there is still the possibility of new life regardless, even as Mélisande falls to her fated death. This pairs nicely with the constant motif of water. When Mélisande loses jewelry in the water, she gains lovers, which is a nice way of saying that she's following her instincts, just as the water flows in the stream. "It's just so natural," one might say. There is a serious thirst for novelty expressed in the story as well, which is resilient and optimistic, but also naïve and immature.
Therefore, Mélisande fails in her greatest goals by succeeding in fulfilling her desires. Rather than bringing her happiness, though, her temptations bring her death. She likely accepted that risk by making the decision, but when the betrayed husband comes for his revenge, she doesn't stand a chance. Even though she escapes with her life, the child of her affair literally kills her, as if the child's existence is condemning enough for her to finally die.
There are gender questions and feminist questions implied by the story, and it's important to remember that the story belongs to a bygone era with different morals and ethics, but part of understanding complex ideas in a sophisticated, nuanced way is having an anchor in the archetypal motifs, and this is a remarkable description of what affairs and divorce really feel like. They feel like life and death, and the betrayal is easier than we think, and the consequences are always more severe and painful than expected.
