It really depends. My partner and I are both poly and are getting married in the near future, while looking for a girlfriend. So if things ever worked out seriously with someone else, it would be done. Would I do it in a situation like that? Yup.

I put depends because I'm not really sure what you are asking. Are you meaning have sex with both partners (i.e. - male and female) with just your lonely self? Or swinging? I think we need more elaboration on this, call me crazy, but I'm lost!

That depends on the situation. My current play partners are married to each other. I would say that having intercourse with a married couple is just fine as long as all participants are aware of their place in the relationship and that the relationship is occurring.

Anything once!

nope i wouldn't do it

Nope.

Nope, I'm not into sharing.

I'm fine with sharing. i'd eve bring my lover along ^___^

Depends on the couple and their situation (and mine!).
Right now, I have a female partner who's married to a man and I only see her on the side (but am friends with him - just no sexual connection). I have friends who are a married couple and I've been with both of them but separately (approved by both of them, of course. Communication is key). I've also been the "third" with a married couple a few times, but only for play - we weren't friends outside of kink.
And ONE time, when I was much younger and more naive, I was with a man who swore up and down that he was in an open relationship. I guess he didn't run it by her, since I ended up with a black eye! I talked to my close friend (who is part of the married couple in which I've been with both halves) and she gave me the simple advice of "always talk to the other half before play". Lesson learned!
So yeah, it depends.

No. We aren't into having sex with other couples or individually with other people.

Not me I'm just a 2 only person - me and my hubby.