A stag fag is a slang term for a woman who either associates mostly or exclusively with heterosexual men, or is best or good friends with a straight man or men. It is a derivative of fag hag and stag hag.
Most of the time, stag fags are homophobes and are usually uncomfortable around other gay men. Like it's phrase-predecessors, it is widely acknowledged that a rich relationship can develop between a straight man and a gay man without sexual tension. The stag fag relationship allows the participants to separate intimacy and sexuality.
Oh no, M. and Y. aren't together. M. is straight, Y. is gay, but he's a self-confessed stag fag.