Do your best to communicate what the game is really about. The "swords and sorcery" genre has never been very inviting to women that aren't already dedicated geeks, but the core gameplay mechanics of MMOs are actually quite appealing to women, even non-gamers: investing in and advancing a character, interacting with people and building social relationships to accomplish goals, exploring a world, etc. So ultimately its on you to communicate the experience to her in a non-threatening way, then let her try it and see if she likes it. If she does, great, if not, c'est la vie.
For my part, I met my wife in an MMO - by far the best way to find out if she is interested in sharing your hobby...
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