me and my wife have a paperback copy of the satanic bible (we were intrested to see what kind of stuff it said, we dont' actually worship satan.) and actually it makes alot more sense than the bible. one of the main things i liked, was about sex and went something like this:
1. must be consentual
2. can't violate vows of a marraige
3. both are of a reasonable age
(these aren't the exact words, been along time since i read it and i don't have it handy or i'd type it word for word, but it gives you the basic idea)

so basicly
any sexual contact is fine, as long as it doesn't break the vows of marriage (so no sex outside of the marriage) as long as it's consentual, meaning not raping ect ect...(which would also mean if all three parties of a 3-some are consenting it's ok, or you may have sex with the same sex as long as both are consenting and so on) all people involved are of reasonable age (take this part how you want but in my eyes it basicly says, you can't do anything with kids)

now don't go jumpin on my ass saying "the devil played off your sinning nature to trick you into being with him" or "that book was written by an insane man" or anything of the like. i don't believe in either, and quite honestly both books were written by men, just at diffrent times.