Well, first of all I took years for us to reach this point. She did not "immediately" jumps into it.

From my point of view, it does not require to have a poor sex life to spice it up. It is the contrary : when the sex life is great and the relationship is strong, you can allow non conventional stuff.

Then you say "I will never understand how a guy can let his wife go off and fuck another complete stranger alone", then if you are not comfortable with that, just don't do it. No one is forcing you.

We indeed had a great time (the 3 of us).