it's mostly used in taiwanese, which means "man whore".

In a Chinese legend, 牛郎and织女（zhī nǚ ) are a pair of lovers. Because 织女has similar pronunciation with妓女(jì nǚ) (female prostitute), so recently people use织女for the female prostitute and 牛郎 for the male pro... Not easy to explore modern slang in varies classes. Good luck

it comes from the movie title 《午夜牛郎》for 《Midnight Cowboy, 1969》http://www.mgm.com/view/movie/1251/Midnight-Cowboy/