You might want to deal with it before you move in, simply to lessen any possible embarrassment or discomfort. Does she know for certain that you are not a lesbian -- any chance you could've said anything that might have led her to wonder? Or, similarly, is there a possibility that she's just a super-friendly, flirty type? I have plenty of female friends who are neither lesbians nor bi nor bi-curious, but they do enjoy flirting with me pretty regularly. I've just come to realize that it's only flirting and would never transcend the hypothetical. Your new roomie could be one of those. But if you really want to start the year out on a clean slate, with no weird baggage, drop lots of hints about guys you're dating or hope to date -- or better yet, come right out and write to her: "Hey Sappho, I am totally cool with your predilection, but just so's ya know, I don't swing that way. Capisce?"..

The same way you'd deal with it if she weren't living with you (or if it was a guy friend who was flirting with you) -- just tell her politely but firmly that you're not interested. You could mention that you're flattered. You could tell her that you don't swing that way, so to speak. She ought to respect that, and soon enough she'll find someone to get involved with...