In this case, the distinction is unlikely to be relevant. The subjects are "women" (unqualified) and the audience is everyone.
If a woman whose gender and sex don't match is interested in being a subject on the site, then the distinction might be relevant. Judging by the tone of the site, they seem likely to be inclusive more than exclusive so long as your story is worth covering. But that's hard to say for sure and certainly use of the term "gender" (unqualified) doesn't tell us anything.
Although a person with male genitalia who looks like a man and acts like a man but claims to identify as female may be denied coverage, the odds of this scenario happening are very low, so the distinction between sex and gender is a non-issue in this case unless you have a particular agenda to push.
Look... I support trans rights. I live in San Francisco. I have trans friends. But call me old fashioned, we already have a word for "cis-woman," the word is "woman". A lot of people really like that word and don't want it changed, and that doesn't make them a bigot.
EDIT: And from a practical perspective, I think the use of "cis-woman" is harmful to the trans community. It makes them seem kind of like they have an extreme agenda, when they really don't. If trans people really hate being labeled as "different," just co-opt the word woman. Stop labeling yourself AND "cis-women" differently.
Assuming the best of you, let me give you a small piece of advice. Never say this. Even if it's true, and even if it's your trans friends who Made You See The Light, etc. Not only is it an irrelevant fact, but it's a very frequent cover for bigotry ("I have a gay friend!" "I have a black friend!" "I know lots of women!") and will never help your case.
There are a couple reasons for this. 1, it obviously didn't keep you from making whatever mistaken statement you made. 2, just because someone is part of a group does not automatically make them sainted evangelists of that group. In stricter terms, it's an argument from a false authority: your friendship with a small group of individuals does not make you (or them) an authority on the larger group under discussion.
> But call me old fashioned, we already have a word for "cis-woman," the word is "woman".
Who is "we" and why do you assume you're included? Language drifts, and it drifts most particularly in subcultures. People discover needs to describe things that they do not immediately have words for and begin using new words. You can be as old-fashioned as you like; just don't prescribe how other people talk. You ain't no po-po for da lingo.
> If trans people really hate being labeled as "different," just co-opt the word woman.
They did. That's when you didn't hear about them.
Think of it this way. A lot of people currently believe that gay people do not exist. I'm serious. They do not exist, and that is part of what justifies legislation against them. They're not trying to oppress people; they're trying to convince supposedly gay people to relinquish their traumatic past and return to the straight and normal. Laws that oppress a non-existent class don't really seem like oppression. Forcefully reminding us that they are a class and that they are treated differently right now has value.
I mean, really. How exactly do you expect them to talk about the issue of bathrooms without bringing up the transgenderedness? And how do you expect them to talk about their transgenderedness without sounding like it's something deviant?
If this is how you talk to sympathetic people, how are you ever going to convert the hostile? Work on your message. And at least for the next 5-10 years until everyone has one, cut some slack for the line "I have trans friends." Maybe someday that will be irrelevant but I don't think it's irrelevant today.
You know why the word cis-women grinds my gears. The prefix sounds like "cyst" every time I read it. It's not too late to pick something else, I doubt more than 5% of the population has ever heard it. Marketing matters.
Don't people know it's the same thing as green? What's the point of slicing such fine distinctions?