sounds like you do love your wife. i wouldn't push her too hard. i'm not really sure what to say, except that, mb she needs to feel that she's #1. to her, perhaps the fact that you are so urgent to have another woman to swing with or be poly with, to her feels like a slight, that she's just not enough.
if i were you, i'd focus on her only for a while, and shower her with positive reinforcement & tell her how much you do care and love her and why...only if it's sincere of course. over time, she may come around once she feels secure.
also, this "tabu" may not be able to be undone. find out what it is that she finds applaing about this lifestyle. just listen, don't debate. good luck.