To declare yourself a "Milk Bro" with another buddy, the act of intercourse must take place with a female on the same or separate evening with the lack of a contraceptive. Note: Filling her tank is a crucial step in deeming yourself Milk Bros.
You slept with _____ ?? So did I bro ! Did you wear a dome? Nawh me neither ! So you filled her tank? ..... High five milk bro ! We're Milk Bros !