with women, you can make any woman lactate by sucking on her nipple for a while every day for a few weeks. try it sometime with the wife. i'm not sure if it's possible with men, but frankly, i don't see why not. when men take testosterone, it causes the body to jack up estrogen production, which makes them grow breasts (like Bob in Fight Club)...i don't know if Bob could've lactated, though, but judging from that article, he might've been able to if he had someone to suck on his boobs...
A good question. Although this is only the best theory...scientists believe that men have nipples because inorder to be most efficient, the process of making humans starts with the same template. Only later in the process do sex specific body parts develop. To use an analogy think of being a home developer and having two models. It would be most efficient if you had a basic start model and only later the client could 'customize' it. This would allow you to have the same specialized trade people work on the houses (specialized labour) and only a very small part of the process would be involved in differentiating them.