Well Eva and Ash had a friendship that ran deep and further back than her relationship with David. So that certainly adds some context in my opinion. Also, Eva doesn't necessarily express interest in other women outside of Ash, she can be coerced into a threesome, but she isn't exactly thrilled with the idea. Additionally, there is some trepidation from Eva, they have several conversations about that night before it escalates to sex. Even after Ash and Eva have sex, Eva is conflicted about how it will affect David.There wasn't any indication of Eva(GGGB) being bi either but one kiss is all it took to change her sexual orientation, to the point she was willing or dumped her long term boyfriend to be with Ashley. Not to forget Ian/Minerva situation, I doubt anyone expected that either or atleast to happen that soon and out of nowhere but it did, so you never know.
What I'm trying to say is that you should always expect the unexpected when playing an EvaKiss game.
So all of this is to say, it's a lot more nuanced that I think you're giving it credit for. Because it was a porn game it felt like it happened very quickly, since EK isn't about to write all of the actual nuances and conversations that would happen in real life if a kiss like that awoke something in two people.