Jesse Bering explores it:
I’ve never seen myself as a “fag”although I’m sure many other people do see me this way and unfortunately nothing morebut more importantly I’ve certainly never regarded my many close female friends as “hags.” So I was curious to learn more about the unflattering stereotypes lying at the etymological root of this moniker, which describes straight women who tend to gravitate toward gay men. Enter Mount Saint Vincent University psychologist Nancy Bartlett and her colleagues, who just last year published the first quantitative study of fag hags in the journal Body Image.
...The results were analyzed to test the common assumption that women befriend gay men because they have poor body esteem and feel unattractive to straight men. If this were true, the authors reason, then there should be a meaningful statistical association between a woman’s number of gay male friends and her body esteem and relationship successin other words, the more pathetic a woman’s romantic life and the more she sees herself as being undesirable to straight men, the more she should seek out gay men as friends. But the data revealed otherwise. In fact, with this sample at least, there was absolutely no link between a woman’s relationship status, the number of times she’d been on the receiving end of a breakup, or her body esteem and the number of gay male friends in her life.
Some girls just like to have fun, without the sexual tension. Vaughan Bell adds:
I wonder whether the disparity between the marking of 'fag hags' and the lack of similar names for men who hang out with lesbians at least partly reflects the fact that gay men have traditionally been more stigmatised than gay women, and hence there is a greater drive to stigmatise those who socialise with them.
I also wonder the situation is simply less common although I can't find any research that has actually looked at the issue.