She has reconsidered ... completely her call on this and you getting upset about it will only drive the thought and desire further away from her.
IF you truly want this to happen, you need to boost her sexual confidence and you need to provide her the reassurance she needs so that she has zero reservations about the marriage falling apart afterwards.
The greatest factor in all of this is YOU. What you are overlooking right now is the fact that women value their relationships more than men do. EVERYTHING they do they act upon after they have gauged the impact it will have on their marriage. Right now, she is feeling that there is a threat to the relationship, or she is not feeling sexy enough to be naked infront of other men.
While I understand your desire and why you want her to experience this, the greatest thing you can do is nothing at this point. Let the thought if this go for now and work on building up your wife's sexual confidence. Just like seasoning a football pla
Take her out more often. Women that go out gain some sexual confidence because their man wants to be seen with them in public.
Take her to places where she can "accidentally" interact with men of her choice (if she likes muscular guys, join a gym ..... if she likes to dance, go to night clubs more often).
Compliment her often ... take notice of the smallest things and point out the things you notice, or the changes she makes. Find something positive about what she does and ALWAYS find something sexy about her.
Spend time with her doing what she likes. Yes .. shopping. If she wants new clothes, buy them. Buy sexier things than what she has now. If she is still wearing "grannie panties" throw them out one pair at a time and replace them with lacy / skimpier articles ... ALWAYS buy matching bras and panties ...
Learn her sizes and "her style" and buy her a little surprise every now and then. Stockings, shoes, shorter skirts, lower tops, see through clothing or new jewelry .... bra and matching panties. She will feel appreciated and it will further your devotion to her (in her mind). "Actions speak louder than words ..."
The Gang Bang seed is planted ... fertilize and let it grow on its own. If you want her to focus on HER more then you have to remove most of the distractions she has. Give her some time to rebuild and she will bring it up again ... AFTER she brings it up, then you can move forward. If she never brings it up, then it was never meant to be .... or she has decided that maybe she wants to take it on one **** at a time. Maybe she can **** another man ... gauge your reaction to that, then (eventually) work up to a threesum, foursum or more.
It may take a year ... or longer. Be patient and be consistant. If it happens, it happens ... The ball is in her court now.
It is wrong, the sooner she stops the better.