My Ántonia explores social issues in a small town in Nebraska around 1900. The central social conflict arises in the form of racist attitudes toward the immigrants who make up a large percent of the population in the growing West. The immigrant girls, who come from farming families, face prejudice from the girls of wealthy, merchant families who live in town. A class system is set up in which the immigrants work as "hired girls" in the houses of the merchant families, and are essentially seen as second class citizens. The issue gets more complicated when you throw sex into the mix, since the town boys place the immigrant girls on a sexual pedestal but aren't willing to marry them.
Social barriers are the ultimate reason Jim does not pursue Ántonia romantically.
Social barriers have nothing to do with Jim and Ántonia's relationship.