Ever heard of ancient nursemaids or wet nurses? Well, royalty may have birthed the babies, but in those days high society had these nurse maids on retainer to breastfeed those royal babies. Though the reasons for such ranged from being unfashionable to infertile, there's more to the skill than meets the eye.
Once a woman is pregnant and gives birth and her breasts fill with milk, if a baby keeps sucking on their nipples, the breast will continue to replenish itself with milk. This explains why some sick fucks can breastfeed their children until they are 9 years old. But then again they've gotta withstand some pretty hefty bloody bite marks to their areolas, so maybe they've earned it. If you let a baby suckle and/or attach a breast pump on any breast every few hours over several weeks, that breast will most likely eventually lactate. That goes for both men and women. That's right, solely nipple stimulation can create the milk producing hormone prolactin, and enough prolactin and milk will be flowing.
From an evolutionary standpoint, it would make sense that humans and other animals would evolve to be able to aid in the survival of our healthy viable offspring should their birth mothers perish. Lately, with breast pumps and formula a plenty, a life or death hand off to a man's nipple hasn't ever really crossed anyone's mind. Studies have shown that despite the gender source, the quality of the breast milk remains relatively the same, quantity, however, is still up for discovery. In closing, the real gender inequality atrocity here is that a man's nipple can be posted on Facebook and Instagram, and a woman's cannot.