As a Cantonese speaker, the mythological meaning of 牛郎 is the first one that comes to my mind. Outside of its mythological context, I can only see the word refer to male prostitutes in Japan and maybe Taiwan.
According to this Wiktionary page, every Chinese-speaking region has its own euphemism for "male prostitute", with only Taiwan using 牛郎: https://en.wiktionary.org/wiki/%E7%89%9B%E9%83%8E