I think most agree (wives especially) that
anal sex is a
little more... private/personal. Even in alternative life styles. My Vietnamese wife has always been adamant about that, even though we Swing and often have MFM with another male that we both agree, should be a stranger to us.
We've done spit-roast and DP, with me always being the one who was in her ass. She was always fine with that, no hesitation. Always took the other male in her pussy or mouth.
On a particular horny MFM session, while she was blowing me, I told the other male to "Take her ass". She looked at me stunned and I immediately reassured her. "Honey it's okay, think of it. This will be the first time a different cock is in your ass. A new sensation. And you'll be proving to me what a beautiful slut you can be". (She does roleplay that a lot)
I looked at the other male as to give him instructions to ease my wife's concern. "He'll go slow. You know you have to relax so he can get in. You've always been my fuck doll. I know you can do this. Once he's in you, you'll be your husband's whore. You've always wanted that didn't you? I'm going to let him cum in you, so he's going to ass fuck you like I do, concentrate on that. When you feel comfortable you can work on my cock to keep me hard".