it all comes down to how secure you are. if you can handle it, then fairness dictates that you give her her fantasy. if you cant handle it, let her know, because itll destroy the relationship if you have to watch it and hate it. either way it comes down to:
1) dont do it -> unfair -> resentment from her, esp if you want to keep doing the 2 girl thing -> undermine relationship
2) dont do it -> tell her that swinging ends now because its unfair on her since you cant handle it -> ruin your sexlife? increase your emotional bond? subjective.-> ruined relationship OR _> relationship goes to another level
3) do it, insecure -> worse insecurity -> undermine relationship
4) do it, secure -> swinger lifestyle -> are you comfy in a long term (married) swinger lifestyle?
personally i wouldnt do it. part of my character is to fall into classic machoism when in long term relationships, and i dont at all like the idea of the mother of my children having being used as a fuck toy to that extent. im just not that kinky.
you should have seen this coming though.