R. Shimon says (on "the loved one and the hated one"): Scripture hereby apprises us that he is destined to hate her (the captive woman) and love another.
"and they bear him sons": to exclude (from the halachah of bechor) an instance where we are in doubt (as to whether the son is a nine-month birth to the [possible] first [man who cohabited with her]), or a seven-month birth to the second.
"and they bear": I might think, from the place of "bearing," to exclude a (son born by) Caesarean section; it is, therefore, written "sons" (in any event).