I come from a community where the female to male ratio of gamers is pretty close to 50/50. Our entire lot met at a fantasy boffer LARP for the most part.
If you want my opinion, I think a lot of the gender gap has to do with exposure. For a very long time (and perhaps still is), tabletop RPGs were never really the "cool" thing to do. Even less of a game that woman were shown to participate in. Before the convenience of virtual tabletops, the means of getting introduced (as a male or female) to gaming was pretty slim - a high school club, a college dorm, a gaming store, like minded friends, etc. The tricky part about being female and going with some of these options is you get a lot of unwanted attention. You stick out. A lot. And you might get hounded for it. This doesn't really help get yourself acclimated to the community and might scare you off entirely.
I was introduced to tabletoping via the gateway drug of LARPing. Fantasy combat and costumes? Oh snap! Hell yeah! Get me some of that! I love fantasy. I love costumes. Met like-minded people there (and my husband) who roped me into White Wolf's Mind's Eye Theater. Then eventually into tabletop games.
There's also something to be said about rules systems. I love to roleplay, but dear gawd do I find D&D/PF/d20 systems really overwhelming! Back in the day, that's what my husband and friends were trying to get me to play and admittedly it was a struggle for me. If you're trying to acclimate a female friend/girlfriend to tabletop gaming, keep in mind they might be getting lost in the rules. Honestly, scratch that, this isn't gender specific advice. If you're trying to get
anyone into gaming, be mindful of the ruleset and if it's causing a stumbling block for enjoyment. Not all rulesets are meant for everyone.
I'd recommend starting out on one shots too. Trying to bring in a newbie blind on a long running campaign might feel off putting because they're going to be reeling by the catch up needed to figure out what's going on. Not to mention they'll be lost when the in-jokes inevitably crop up of past misadventures.
If you were to ask me, just adding women to the table doesn't change the dynamic of it. The table is directed by the individual idiosyncrasies of the players sitting at it and doesn't have anything to do with their gender.