I don't think they're few and far between, but they are still a relatively small number, particularly when you limit it to novel writers, and especially when limited to writers of the sort of SF novels I like to read.
Which is not to say I'm not a fan of any women SF writers, but I don't apologize for the fact that most of my favorites are men.
Don't apologize for your tastes. We all like what we like.
I'm just saying it's a shame if you're not finding writers (of either gender) you might enjoy because they're not being reviewed, promoted, blogged about, talked about, put on display in book stores, recommended to friends or on fantasy and SF-focused boards and so on as often as their male counterparts.
While discoverability is an issue for all new writers these days, there is some evidence that it's more of an issue for female writers. Hence the (mis)conception that female writers of SF and F are more of a minority than they actually are.
Around 45% of the novels received by
Locus in 2012 are written by women.
http://www.strangehorizons.com/2013/20130422/2sfcount-a.shtml
The number that sticks in my mind for female SFF writers is around 40-45% or so of the total number of SFF
novels being published, with the numbers a bit higher for fantasy and a bit lower for SF. There are also differences between the US and UK, with the US having more women SFF authors overall. I'm having a hell of a time finding that reference now, however. I think it was an article on the SFWA site (or linked there), but eee, recreating the exact wording of google searches from months ago is not my forte.
I found
one that was specific to Australia, which puts female writers of SFF to be over 50% overall, with a higher percentage of women writing fantasy (62%), and a higher percentage of men writing SF (57%). Horror seems to be heavily male (82%).
Of course, there will be gender differences between subgenres. If you mostly read harder SF, you may have fewer woman authors at your disposal. Maybe only around 30-35%. If you read UF, you may actually have more female writers. If you read secondary world fantasy, there may be a more even split.
But if you think women are only writing 20% of speculative fiction novels overall, then I think it proves my point, since they do write far more than that.
There are some lines of evidence that female authors are not getting promoted and reviewed in numbers that are representative of their participation in the field. This would present a barrier to discoverability.
http://thewertzone.blogspot.com/2014/02/waterstones-fail-to-recognise-female.html
http://www.strangehorizons.com/2013/20130422/2sfcount-a.shtml
http://www.sfwa.org/2011/06/guest-post-checking-the-gender-balance/