I would agree with your last statement.  My wife and I are both bi and I think it is a huge factor that enables this type of lifestyle. 
The three lovers that we both developed feelings for were bi, as well.  I don't seek out men, per se, but sucking my wife's lover's dick, on occasion, is very enjoyable. 
My wife loves it when I do that.  To her, she says, it shows my complete acceptance of her lover.  To me, it's an honor thing.  It's the best way for me to show how much I appreciate the man and cock that brings so much pleasure to my wife.