The Mafia was a family. It is literally “A FAMILY”. The Godfather is literally everyones GODFATHER.The relationships were very close. It was so tight, that betrayal is treated like you hurt a member of the family. When a superior kisses a person in the family(only one his own), it meant to represent The betrayal of Judas against Christ. Thats where it originated from .It was done in many cultures . Upon seeing the very public kiss, everyone knew that person was next to die without talking about it. They were marked.
And all this time I thought they were homos. Go figure.