Will a threesome with my wife and another lady ruin my marriage?

Not in and of itself.

And I imagine it is a fairly common fantasy for straight men at sometime in their lives.

Prior to 1980, Mount St Helens in Washington state was a scenic beauty.

But when it erupted on May 18, 1980, those who were actually close enough to "participate" in the event probably at least momentarily regretted their decision.

You are playing with fire---never know how that is going to turn out: Controlled burn or holocaust.

Not if all parties are agreed to the party.

Don't even think about it. I've known of three cases (well, four) and all ended in the marriage breaking up.

The first was two men and a woman, and the woman liked the new guy best. Bye-bye husband.

The second was two women and a man. (I used to run with the second woman who told me all about it.) The two ladies liked each other. Bye-bye husband.

The third was a neighbour who liked to share his wife with various men. Inevitably she met one she liked better than him and, well, that was another bye-bye.

The fourth was actually two couples sharing. They were mature, middle-aged, and had been doing it for years. They were good friends and there didn't seem to be any friction. I haven't seen them for years but the last I heard they were doing fine.

Anonymous, it may look like fun when you're watching naughty movies, but I can't think of anything more likely to ruin a marriage.

Um yes it would ruin it. Women want respect

If you can, without a doubt, predict how you both will feel AFTERWARDS .. Sometimes, how you feel can change after the fact. You know .. It's kind of like .. " Well it SEEMED like a good idea at the time".

It could plausibly create doubt and mistrust and shake your relationship to the core ... If you think what you have now is worth taking the chance .. Then fill your boots.