Because she does not currently fancy men at all in any way. My experience is that quite a lot of gay people have heterosexual relationships at some point because there’s an awful lot of social pressure to do, but this doesn’t necessarily mean that they’re all secretly bisexual. Of course, that’s problematic because it supports this idea that bisexuality doesn’t exist or is somehow cheating. Basically it’s complicated and, in this specific instance, I came down on the side of a homosexual character who has had one arguably heterosexual experience rather than a bisexual character, but I think either would have been a valid call to make.