She's not in denial, nor homophobic. The thought just never crossed her mind because it's out of her reference framework. Like most people, she's probably not been confronted to gay/lesbians in her life.
For a lot of people, "gays" are something alien you see on tv or in the news, not in your neighborhood. And even less as your roomate.
At least as importantly, she's never had a meaningful relationship that's turned out well. The closest was her last one, and then the miscarriage and general horribleness that came along with it happened. I doubt she even has a reference point at all for "supportive, healthy and enjoyable romance".
Before getting into bullshit theories about homophobia, don't forget the obvious, story-supported things staring you in the face. And for that matter even referenced in the most recent chapter.