Take the emotion out of it, picture that your friends are asking you for advice on the same topic. Really think through it. Not just the sex act but after that. How your friend would feel after the fact. How she would feel about what she did, how her husband felt about it, how she felt about her husband. The jealousy. The emotions that would get stirred up from the third party, your friend, and her husband. How it would lead to other things. What effect it would have on the marriage and their regular sex life.
Spend a good 15-20 minutes just thinking about how the marriage would look AFTER the act was over. The realistic outcomes. And then tell us that for even one second you think that this could in any way be a rem 6otely good idea.
Keep it a fantasy, roleplay if you have to, but don't be an idiot. For every one marriage that makes it through something like that, there's about 9,999 that end up divorced or miserable. The small payoff during the act is never going to overcome the myriad of negative consequences. It is much like drug abuse. First hit is great, everything after that is just chasing until you hit rock bottom.
I'm really curious where you got you stats from.
As an FYI, my GF and I had a threesome/ foursome this fall, after talking about it for awhile. And we had a fairly painful discussion after the fact. It's one thing to think about seeing your partner with someone else, but quite another when it really happens. In the end, I think we came out stronger, and plan on trying it again, but there's definitely risk involved.
And keep in mind that this was" just" with my GF. I love her an awful lot, but we've only got about a year invested into each other, and no kids, mortgage, etc. Would I gamble a marriage on it? I'd have to be pretty damn confident with our communication and security of both of us. I was very comfortable in our communication, and we did work it out, but it could have backfired.