The idea of marginalization can be seen in a couple of ways in Bontemps' story.  On one hand, there is a social marginalization of the Pattons that has happened.  Being of color, relatively poor, and of color in a setting where racial stratification plays a strong role in the narrative have all contributed to ensuring that the voices of Jeff and Jennie are effectively silenced.  This marginalization contributes to the idea that ending their lives is the only answer because there is little in way of progressive hope in this vision.  At the same time, there is an emotional marginalization that happens because of the social level and outside of it.  Jennie and Jeff have endured the challenges of losing their children in a small window of time as well as their own physical debilitation.  In a sense, this has played a role in their marginalization from being in the world.  There is a love between both of them, which compels them to undertake their plans.  In this setting of marginalization, the silencing of their happiness or potential for joy is what drives them to embrace the "inevitable conclusion."  The marginalization or silencing of voice that happens to Jeff and Jennie happens on these levels, acting in concert with one another.