Mine are hardwired to my cock no doubt about it, and oddly enuf the response is greater the harder they are squeezed. I can be really close to the edge but just gripping my cock nice and hard: if my nipples are then squeezed very hard, especially if the grip starts out lightly and gradually increases, it will bring me to orgasm without additional stimulation. There is a certain unmistakable point of pressure that sets something off in my cock.
It also enhances my erection if, instead of the hard grip, my wife lightly flicks the erect part of the nipples with her finger, occasionally scratching them lightly with her fingernails. I think nipple play is one area that most straight men ignore, thinking it's too gay, but they could really have stronger orgasms if they'd open up to it.