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.
I do not know if Keiko is in denial, but even where we saw Keiko ever felt like kissing Ran. chapter 12 and chapter 30