I think what I'm trying to say is, I wish we didn't have the drunken mixer as a way for them to have slept together and instead just have them idk flirting on the bus or something
There are truckloads of stories out there where one or both of the girls are drunk when they sleep together for the first time. It's like the oldest plot device in the book.
Sometimes, she's a timid little thing who needs some liquid courage to dare do what she wants to do.
Sometimes, her head is full of repressions and inhibitions, and she just doesn't understand herself -- till the booze washes away the heteronormative conditioning and brings out the real her.
The important thing is that, next morning, she wakes up to find another woman in her bed and is forced to face the fact that she likes doing it with women (way more than with guys, in fact).
If you disapprove of this storyline, you'll have to forsake an enormous chunk of the best yuri series out there. Just sayin'.
(BTW, this particular manga is not an example of this trope. College girl was already sober when they left that first diner, and she went to the other woman's place fully expecting to have sex. She's all like "omg are we going to shag or did I misread the clues?" up to the point when the other lady tells her in an unmistakable way that she wants to do her -- and she answers that she wants it even more.)