My experience is that nipple stimulation causes arousal in most men, although gay men tend to be more inclined to experiment with this, or to admit the connection. In some men, there is a very direct conenction between the nipples and the penis.
My nipples are fiarly sensitive, yet very tough, and connected to my cock. The boys have disovered that working my nipples hard will soon get them fucked. As such, my nipples are now quite enlarged from all the rough play.