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.