For one thing, the 'straight' girl just slept with a woman without us seeing the mental gymnastics she would have to go through to allow herself to love another woman. If a lesbian kissed a girl who's only liked guys before, she probably wouldn't just suddenly throw her identity out the window without some serious thought.
Eh, sometimes these things happen and auto-pilot kicks in. Sometimes those thoughts don't come until the day after, and some people it clicks with and there doesn't have to be huge questioning. Also, she may just have already figured out that she's bi and was only shellshocked because of what had been sitting under her nose. Who knoooows!
In addition, I understand why people would see the other woman is taking advantage of her sadness. It's not enough to put her in 'rapist' or even 'creepy' territory, but it's something to be aware of.
Yeah. Someone neurotypical being taken advantage by someone else with no particular authority over them doesn't have to be an awful thing. It isn't admirable, but I don't think we should be upset with Aki for seizing the moment. Policing consentual sex seems like an absurd ask outside of situations where someone objectively can't understand what's happening, and she very obviously is lucid enough reciprocate.