I think the reactions really depend on if its a couple and another couple, and if both parties really do want something as opposed to one of them participating after a little coaxing.
I can see how resentment and insecurity would fester should say, the woman, only have participated after a little convincing. But if both people want and fantasize about it equally, and I think if the 3rd person isn't a regular part of their lives, it's a little different. There's no during-sex "I don't like her touching him" or post-sex "did he like her more than me" if, for example, you both mutually brought her into your bedroom to sexually satisfy both of you.
I also don't think it necessarily means people who pull it off are detached from sexual intimacy in general. I think two people could be very much attached to each other without attachment to the 3rd person.
I think for sure the determining factor is the mutuality of the fantasy, even more so than the trust or security, though those are both musts as well.
But I'm talking about something slightly different. I wouldn't bring in another couple.