You're not going to like my answer Elijah. My wife learned long ago how to ring my bell. A little nipple play will almost invariably produce an erection, and this is true of many men, though they often don't like to admit it because they perceive it to be effeminate. By contrast, my wife has never particularly liked me to reciprocate with her, and again, this is not unusual. Studies have found that sensual/erotic/erogenous qualities of nipples varies more from individual to individual than from one gender to another.
This is not surprising really when you consider that all breasts are rudimentary the same, starting from the same embryonic tissue and have the same nerves. At puberty, hormones cause the nipples and mammary glands of females to enlarge and develop, and adipose (fat) to develop around them to make up the bulk of female breasts, but the nerves are basically the same in each, even the erectile muscles are present in each. The same things that will cause a woman to get erect nipples will cause a man's too as well.. It is just less notice since they are smaller, but if any woman doubts this- try it on your husband- if he will let you. Many men won't.