Honestly, I think things are better this way and I hope the ship has sunk (sorry Ishigami x Tsubame shippers). I don't think this is the end of Tsubame in the story because we still need to get to delve deeper into her thoughts and motivations regarding what happened this chapter because I feel like there's more to it and there's still time left until the deadline, but I don't think she and Ishigami will end up together. Why? Because regardless of Tsubame's level of interest in him Ishigami himself considers himself beneath her. He puts Tsubame on this pedestal without really knowing her (and I think on some level Tsubame realizes this). Simply put, they're not on equal standings in his eyes, and no relationship can work with that kind of mindset. A long distance relationship even moreso. Because in a relationship you're equals. You're supposed to be at least.
This is why I personally think Iino is a far better match for him. They're much more on the same wavelength than Ishigami and Tsubame. They also complement each other very much, almost too much actually, and I've never once felt that Ishigami and Tsubame complement each other. At all. They're just too radically different (arguably so are Iino and Ishigami but I digress) and are in completely different social circles/standings overall. I think Ishigami has a much better chance at happiness and a long term relationship, perhaps even life-long, with Iino. They've always been very cat and dog, but they've always cared for each other deep down, understood each other very well and stood up for/helped the other when it mattered and that's what counts. Iino recently gained a whole new perspective/understanding of Ishigami too during the bonfire, and her behaviour and demeanour around him has changed a lot since then, strongly hinting at romantic feelings. I'm very curious to see what will happen between Ishigami and Iino during the remainder of this night.
Then again, this event may have allowed Tsubame to learn more about Ishigami and gained a new perspective on him. He'd rather do it with someone he's in love with, with a person who feels the same way about him and in a committed relationship with; showing he isn't just a pump and dump kind of guy. He wants love.
To the people calling Tsubame a whore; she very obviously genuinely likes Ishigami or she wouldn't have been willing to give him her first time. Everything we've seen of her shows that she's the opposite of a whore. Tsubame is allowed to want to have sex with Ishigami because she likes him without entering a committed relationship with him. It doesn't make her a whore. I believe the only real thing that Tsubame did wrong here was that she didn't properly tell Ishigami that going out wasn't going to work before they started getting intimate. She should've let Ishigami know beforehand and let him make his decision from there. With that said, Ishigami probably shouldn't have confessed right before doing the deed, but he is our boi and a good boi. He wanted to lose his virginity in a relationship of mutual love and commitment. Tsubame may like him and wanted to give him her first time but she didn't want a relationship due to the long distance. Also, long distance relationships are HELL. Rarely do they work out. I don't blame Tsubame at all. She wanted a nice memory before graduation and to lose her virginity to someone she likes and likes her, but that wasn't enough for Ishigami. It happens. It was one of the most realistic depictions of almost-sex I've ever seen in a manga.