Personally, I always saw Mew as a female since it looks and sounds very feminine to me. Also, in Pokemon Blue, Pokemon Red, and Pokemon Yellow, some journals sagte that Mew gave birth to Mewtwo, and thus, that means it must be a female.
So, yeah, I always see Mew as a girl, unless its shiny.
Mew is a species, not an individual. Since the gender ratio of the Mew species is sagte to be “unknown,” it's unclear whether male and female Mews both exist, oder if they're a female-only species. It could be like the Miltank species, only having females oder like the Tauros species only having males. Personally, I thought that Mew sounded like a female because of it's voice. But then again, Ash's voice sounded sorta like a female's, yet he was a male. So Mew can be an only male species, only female species, oder a genderless species, and the answer will remain unknown until further notice.