פירוש על עבודה זרה 70:20

We can see here in this story how contentious of an issue the bread of Gentiles was. Aibu was sort of hiding his eating of the bread, and still Rava severely criticized him. My impression is that this issue was delicate because the only possible reason for the prohibition is the fear of fraternization. Not eating Gentile bread was simply a means of keeping Jews away from non-Jews. It is not like wine, which is sometimes used in idolatrous ceremonies, nor is it like cheese, that might have non-kosher ingredients. It would be more like beer (yum!) which is also prohibited only because of fraternization. From the early medieval period, Ashkenazi Jews were not strict about this prohibition. In times when bread was usually made communally, and large communal ovens were needed, they did not have any choice but to eat Gentile bread. In modern times, as long as the bread is kosher, non-Haredi Jews generally eat bread baked by Gentiles, especially professionally baked bread.