Not that wrong. It doesn't matter how good Rucka is at writing lesbian characters (though, to be fair, I think he's pretty good). He's still not a lesbian. His point of view is not that of a lesbian. And when he writes lesbian characters, he's doing his best approximation of what it means to be a lesbian based on his point of view.
And before anyone comes up with "But he's also writing about Kryptonians and vigilantes and he's not either of those," yeah, I know that. But he is still writing about his point of view of Kryptonians and vigilantes and whatever else he writes about, and that general broad point of view (the straight white guy) is all too present in comics. Representation should be happening behind the scenes as much as it happens on the page, and editorial do themselves a disservice by having him be the go-to guy to write lesbian stories, when there are plenty of talented lesbians out there who'd love the chance to write lesbian stories in comics.
(But again, I really like Rucka and hope he keeps getting work. Take a book away from Geoff Johns to make room for that talented lesbian; dude could certainly stand to ease up on the DCU. :P)