I'd agree with your betrayal of trust if the MC was purely acting on his desire to get laid. But he isn't.
Agawa has completely smokescreened him and he's an inexperienced kid who's just lost his virginity with a girl and has no idea how things should progress. She has attempted to pimp him out effectively and straight up told him he needs to get more experience when he tried to deepen their relationship. It's perfectly understandable for him to believe what she says. She's manipulated him. Hence why people are complaining about the expectation of him being a mindreder. For all he knows, maybe she wants him to try another girl so he can confirm whether he actually likes her or not. That's how she's behaved to him.
I get that perspective. If he believed her and thought, "I
should get some experience with other girls just to get a taste of what's out there and figure out if I'm actually in love or just horny" like a lot of people do, both men and women, then did it with another girl, it would be totally understandable. Again though, I totally get that he's a dumb kid and it would be
totally understandable if he didn't have the emotional intelligence to even consider this, but just considering his own act of confessing, he should communicate that he didn't mean it or that he isn't sure or
something so that she doesn't get the wrong idea that he was manipulating her.
I'm not saying "if he sleeps with someone else at this point, it's purely out of a desire to get laid" but that "if he wanted to (which he doesn't) and he thinks about it (which he has), it could come off as him saying that just to use her, so why not take 5 minutes to avoid the worst case scenario of traumatizing a girl that he at least likes as a friend and tell her it's not like that before doing it?" He doesn't have to be a mind reader, he just has to put himself in her shoes and think about how him sleeping with someone else might hurt her. He would also have to consider the possibility that she's lying or she doesn't fully understand or trust her own feelings which is a high hurdle for anyone, especially a kid in high school, but given his attitude these past few chapters and the way he reacted to her friend's warning, he gets it, even if it's purely through intuition.
Honestly, a kid his age is kinda
expected to fuck a situation as messy as this up. It would be more realistic if he hurt her horribly, albeit completely unintentionally. It would be understandable, expected even, but still his fault if his actions hurt her IMO. Thankfully he seems to have gotten a gut feeling and decided to go the cheesy route of putting his heart before his dick and choosing to be completely devoted to Agawa purely because he likes her, regardless of what anyone says, even what she says. At this point I think I've said all I can on the matter tho, if anyone doesn't like it we can just agree to disagree.
Whether her sister forces herself on him anyways or not is another issue though.... Usually no one believes that a man even
could be SAed by a woman especially if she's hot, but no matter what anyone says or how it looks to Agawa, if that goes down next chapter that's gonna be 100% on the sis.