The Sages forbade eating bread [and other grain bakery products] of the gentile nations as a precaution against intermarriage. [This prohibition is referred to as Pas Akum. The Sages did not completely prohibit the consumption of gentile bakery products and rather restricted its allowance, only permitting it with the fulfillment of certain criteria, such as Jewish involvement in the baking process as will be explained in coming lessons. The reason behind this prohibition is because partaking in the meal of a gentile brings closeness with them, which can lead to intermarriage. The reason the Sages forbade specifically their bread is because the bread is the main staple of the meal.]
The law if the suspicion of intermarriage is inapplicable: The above prohibition against eating the baked grain products of a gentile applies even if there is no suspicion that one may come to intermarry with the gentile’s family. Thus, even if the gentile is not married and does not have any children, and even if he is a priest which opposes marriage and will never have children, his baked products remains forbidden. Even if the gentile is still a child, and even if he is physically incapable of having children, the prohibition remains. Even the baked grain products of a gentile king and aristocrat are forbidden, even though the King will certainly not allow his family to marry with a Jew.
The Kabalistic reasons:
The Ben Ish Chaiy comments that although the bread of a gentile was prohibited due to Rabbinical decree in truth this prohibition contains deep and mystical purpose and meaning. The Arizal was very careful to avoid eating Pas Akum even if there was a mere doubt if it was Pas Akum.
 112/1; Mishneh Avodah Zara 35b
 Michaber ibid; Gemara ibid 36b “The Sages decreed against their bread and oil due to that this leads to drinking their wine, and they decreed on their wine due to it leading to intermarriage, and idol worship”
 Rama ibid; Rashba 248; Shach 112/4; Taz 112/1; Peri Chadash 112/4; Erech Hashulchan 112/2; Chochmas Adam 65/1; See Kaf Hachaim 112/9
 Shach 112/4
The reason: As even if he does not have any daughters, certainly he has friends with daughters, and the intermingling with the childless gentile can lead to intermarriage with one of his friend’s daughters. [Shach ibid in name of Rashba] Alternatively, the reason for this prohibition is due to a Lo Pelug. [Taz 112/1]
 Kaf Hachaim 112/9 in name of Rashba ibid
 P”M 112 M”Z 1; Kaf Hachaim 112/10
 Chukas 2/Hakdama
 Shaar Hakavanos; Hakashrus 19 footnote 9