I've been working on my orcs lately.
So far, I've got it down to seven tribes and marriage rituals. I decided to base the marriage ritual on Malay customs, since the western style of Marriage in America (and England) seems so common place.
You exchange vows and a cleric or a judge pronounces you man and wife. Didn't seem fair to put that spin on a Warlike race, especially when marriages today doesn't seem to be so special. It's easy to get divorce in America, so I wanted my orcs to have some class.
I looked up Indian and Malay marriage customs, and picked the Malay marriage rituals and added the Polynesian bride price ritual of giving a herd of cows to the bride's father. I then added a feast where a bull is slain and the orcs partake in the feast. Eggs are given to most every guest as a symbol of fertility. I could add shades of the Orphic Egg to the Mythology of the orcs to explain the Creation. ("from a cosmic egg laid by a diety, sprang forth the first orc man and the first orc woman. They then had sex, and gave birth to our race.")
However, I love the idea that the orcs believe that they came about as a result of an act of sexual reproduction. A society's religion that teaches that man and woman came from sex puts special emphasis on the family. I may even have tribe justify masturbation by making it into a creation myth ("Gruumsh rubbed his penis and spilled his seed upon the ground, and from the ground sprang Luthic"), thus justifying that tribe's backward ways in treating their females.
However, if you are a world builder, I must reccommend GURPS Religion. It teaches you how to build religions for your world. Since I'm building by ideas incorporated from other cultures, I tend to build mythology around this. But the book has been invaluable.