Two scenarios here that get me hard. First is real life scenarios of other men checking out my wife's tits or getting an upskirt peak of her bare pussy. Seeing how turned on they get always gets me hard.
Second one is that I have jerked so many times to videos and pics other men have sent of themselves jerking to my wife. The shared appreciation and bonding over a pair of beatiful huge tits like hers is an enhancement over any solo session.