I wholeheartedly agree with EvilMJ. She expressed herself in a way that I was unable to do, laying out her thoughts in absolute synchronicity with my own.
Take what she says to heart .... ALL of it.
Short anecdote:
When my hubby and I started out ... kissing others was a no-no. We soon discovered how completely unreasonable and unrealistic that rule was. It was banished soon after our first experience.
Anyone can say what they want about the "purpose" of sex; ultimately the end result is going to be an orgasm. Where that orgasm is placed should really be of no consequence if you are going to be enjoying all aspects of sexual freedoms and positions. My rather crude take on the whole thing is that if you don't want him cumming inside someone's vagina or mouth, then his penis doesn't belong in either place.
Another short anecdote:
Anal is off-limits for us with other people, though we do enjoy it together. This was something that we did not discuss nor communicate about ahead of time, and it led to a major mishap in one of our encounters with another couple. There were were, me with the other hubby, my hubby with the other wife. I was really getting into it and told the guy to do me in the ass!

BAD MOVE on my part. No matter that the guy I was with tried and FAILED (he was having erection issues and it just wouldn't go in), when hubby looked over it APPEARED to him, by our positioning that we were engaged in full course anal sex. My hubby got up, got dressed, and announced LOUDLY that we were leaving. He was PISSED and yelled at me all the way home. We slept in separate beds that night and it took a lot of begging and pleading on my part to try to convince him that it didn't happen. He was also upset that I even *allowed* anal with another person. I explained to him that I didn't know it was off-limits. He said that I should have assumed it was off-limits.

I am not sure if he ever truly believed me that it didn't happen (it didn't!), but we have moved on and now I am completely in the know about what is and is not allowed for penetration. I will not make any mistake like that again.
And I would also like to add that by being a member of this board, a VERY REGULAR member at that, I have discovered that there are things people do that we have never done, and cause me to question whether or not it is acceptable to my hubby if we were to attempt anything. Case in point: allowing another man to come in my mouth ... not allowed by my hubby. But do I care if he comes in another woman's mouth? Nope, not in the least. I have found that by reading the threads that come in, it opens up new realms that the two of us may not have thought of, and allows us to further lineate our boundaries by discussing these situations encountered by others.