Somebody always says that. Tell it to the Clays, the Breckenridges, the Thomases, the Lees, the Crittendens, the Bateses, the Goldsboroughs, the Terrills or any of the other families torn apart by the conflict. Tell it to West Point classmates who found each other on the opposite sides of the battlefield.
Two brothers born in Scotland found themselves in that position:
In the aftermath of the battle (which was won by the Confederacy), James coolly wrote his brother: “I was astonished to hear from the prisoners that you was color Bearer of the Regmt that assaulted the Battery at this point the other day.” He also said that that if they met again, “you have but to discharge your duty to your cause for I can assure you I will strive to discharge my duty to my country and my cause.”
Tell them it "wasn't actually a civil war" ...
I think the OP meant the literal definition of "Civil War", which is multiple sides within a country fighting for control of the government, which it wasn't. The Confederacy wanted to leave and govern itself, making it more closely related to The Revolutionary War (a "rebellion").