I'm going to go out on a limb and say she's not the first, but the first openly. I'm pretty confident though. I'm not a gambler but I'd put money on it.
Point is that you can be closeted (I don’t know what the word is, if there is one, of someone who is gay but so fully repressed that they fully participate in the straight life) and be elected. Married, kids, the whole thing. So the position is that it’s highly likely that a lgbtq person has been elected, just that they weren’t open about it or were “straight”.