I think the male nipple is mostly useful for hanging onto, in a particularly vigourous sack session. Most guys don’t realize how sensitive nipples are and usually stop chewing on yours when you start chewing on theirs. So, if your guy is a little rough, pich and twist his.

http://www.the-spine.com/wp-content/uploads/2007/10/17nipples.jpg

But seriously, the chromosomes that give us nipples are non gender specific. Then, the gender specific chromosome come in behind the non genders and decide whether or not they are going to be functional or non functional.