1. She's bisexual.
2. Her creator and a lot of the show staff/cast/etc./everybody in the biz is bisexual.
3. Lots and lots of women are bisexual, some studies/research even say that most women are bisexual, debate that as you wish.
4. Some research indicates that bonobos are a closer species to our common human ancestors (though just a tiny bit) than chimps. It benefits bonobos to be bisexual because it helps with group bonding and with easing tensions within the group. The single biggest threats to bonobos existence are lack of group cohesion and group tensions because bonobos have NO PREDATORS, the only possible risk is bonobos fighting. Being bisexual does not prevent them from having as many kids as is physically possible for bonobos, bonobos also have tons of straight sex and tons of kids.
5. Once you take away recent human restrictions under threat of force/violence more humans may be comfortable to express bisexuality. Going by other species, bisexuality has zero negative impact on fertility, so there is no reason it would be selected against.