This boy... this boy is lying to himself, claiming he doesn't care about Suzuka's feelings. He's just pushing those thoughts aside for the moment. You don't go suggesting a romantic evening like that firefly thing if you don't still have lingering feelings for the person you can't just brush aside. Everything that happened between Yamato and Suzuka by the Akitsuki home was destined to happen, and a clear indicator of the feelings they both still have for each other.
He's only prolonging the pain for Honoka because he let her sweep him up in a temporary feeling of contentedness. She can only hold onto him for so long before he's drawn back towards Suzuka. It's a shame, because she really is a sweet girl, but she's just trying far too hard for the wrong boy- a boy that'd probably help anyone else if they were in a similar predicament to that shrine bell situation. He's just that kind of guy.
To be honest, that's a really simple thing to fall for someone over- a single act of kindness... but I suppose I've heard worse. I feel like there's more to Suzuka and Yamato than a single act. Sure, there was a simple instigating moment for Yamato to Suzuka, but they've both been building up bonds since before school started- and a lot of time outside of school, too. Honoka had to wait to meet him properly after school started, and even then he was keeping her at arm's length as he continued to interact earnestly with Suzuka.
Honoka decided to jump line, so to speak, and it's going to hurt her in time.