Sharing my wife's cunt and cock-sucking mouth with a hot guy is incredibly exciting. Preferably with a guy we are close to and whom we are both very attracted to. Bi is great, but definitely not a requirement -- he just must be into sharing her, not only having me watch. The whole sharing part is the ultimate male bonding experience for me. A big cock gives it a special kick, but that too is definitely not a requirement.
It feels so fucking intense to get beyond the exclusiveness/ownership thing with my wife's body and her affection and to happily give her all sorts of experiences, and to be trusting enough of what my wife and I have to be able to allow my buddy inside the woman he craves so so much.