That's not how it works. Rin doesn't get a shot by default just because she waited for a long time, and so it's not a betrayal to not give her that shot. She might have expected it to happen, but it's an unreasonable expectation - if I expected Selebus to give me half of his Patreon earnings, it wouldn't be a betrayal for him to not give them to me.
It is a betrayal because she specifically asks you to hold off to give her a chance, twice, and player Sensei agrees in that scene, forming the promise that he breaks to become a bad homie if Schadenfreude is triggered before the beach event.
Honestly, the only way I can qualify this as a "betrayal" in any way is Sensei not immediately telling Rin that you were already finger deep in her crush. "Oh... sorry, I already made a move on her. I didn't know you liked her" or something.
Of course, I can only justify this if you did Chika's events before you did Rin's.
But honestly, this is all just game play of words. Selebus released the fingering Chika event 2 updates before the beach event on purpose. Moreover, Sensei went straight to fingering her in the event without us knowing he would.
Some say "Oh, you pursued a relationship with her!" but I went in wanting to know her story more, not fingering her. I wasn't given a choice between befriend her and fingering her, just one to finger her or not coming any closer to her.
Except you don't have to have already done things with Chika to betray Rin, there are at least two paths that lead to it, touching Chika before ever talking to Rin about her crush and talking to Rin about her crush before touching Chika. Doing things at any point when a promise is made is a betrayal. It is an even worse betrayal if he promises not to touch Chika after already having touched Chika because you are literally promising to let her have something you have already taken. Player Sensei is a rotten bastard who takes what he wants with zero hesitation, this has been well established by now. Literally the only thing stopping him is the fact that Schadenfreude is player triggered, meaning you have to go to its location after earning the necessary affection, so of course there is no option to stop him in the middle of the Schadenfreude event, we are not in control once an event begins, he is. This lack of control is a common theme throughout the game and is actually referenced through the fourth wall at least once, meaning direct shots taken at the player, not the character, for thinking control is in the player's hands. The only way to maintain control is to time going to the event locations with the requirements met when you are actually ready. In this case, that means after the beach event.
By Schadenfreude, it should already be exceedingly clear that with every new event, you are going deeper into a relationship with Chika and thus deeper into breaking the promise to Rin. "Oh, but I didn't know that would be the event Sensei starts fingering her instead of listening to her troubles" doesn't really come into it, it's just a convenient threshold - even if nothing sexual happened, you would still be breaking the promise. I honestly thought that threshold might have been the kiss before I checked it.
Precisely, this isn't one of those games where the girl comes onto the MC from the start, every single girl takes time to build up and that build up is obvious. Also, taking into account how readily available the two warnings from Rin are plus the fact she is in love with Chika, the lack of this information on any player's part should be a non issue. It is a player decision not to pursue Rin far enough to see the warnings and the warnings are in what is literally her seventh event, Cafe Mocha, with no sexual or romantic event between the start of the game and that point. They don't even need to pursue Rin romantically or sexually, if not wanting to do her path is the issue, because there is no romantic or sexual pursuit of Rin at this time. She literally has no romantic or sexual path, only two instances of boobs, one of which is just too creepy a situation for even the grand opportunist, player Sensei, himself to take advantage of and the other being a total accident.
