Just my two cents

Trust me, I'm an advocate for communication in relationships so usually situations like what's happening in Chapter 10 right now grate on my nerves to no end, but I'm more tolerant in this case because believe what Ichikawa Kei is trying to portray here is a realistic issue and worry that is extremely predominant in gay relationships.

Sometimes we forget that being in a gay relationship is very different from being in a straight relationship. What's at stake and what's usually the main concern plaguing each party's mind are entirely different for gays because your sexuality is simply not accepted by society yet. What I've been saying so far may seem obvious and maybe even commonsensical, but many people seem to forget this fact because of how many yaoi manga portray gay relationships as something that is normalized in society, which they are clearly not and the reality is far from pretty even in countries where gay marriage is legalized. It's still a social stigma and there's still a long way to go before it is widely accepted.

Having your supposedly gay boyfriend cheat on you for a girl is different from having your straight boyfriend cheat on you with a girl—and the thought of that reality is terrifying, and this fear is reflected very clearly in Narasaki's behavior and thoughts. It's not just the relationship and your romantic feelings becoming invalidated that comes with knowing that your partner is cheating on you, it's your very gender and sexuality, the center of your identity, being trampled over because you will inadvertently feel inferior to the other sex and that inadequacy is debilitating because it's not something you can just change on a whim and at your will. That's how crushing it can be to find out that your partner prefers the other sex over you.

Therefore, it's entirely understandable for Narasaki to be apprehensive about confronting Terashima about this because he would have to question everything about himself—his feelings, his self-worth, his right to love—if he were to find out that his insecurities are actually true. It's human nature to want to avoid pain and the truth, especially when it hurts—and for someone like Narasaki who is naturally taciturn and introverted with an innate propensity to avoid direct confrontation, it's doubly difficult for him to speak up about it.

But hopefully, everything will turn out alright soon and the unspoken misunderstandings are finally aired out to be discussed healthily.

EDIT [SPOILER-ISH]: I'm glad to say that after reading the raws, Narasaki and Terashima will talk about their relationship issues like level-headed, responsible young adults and reconcile with each other! Thank god the silence and misunderstandings aren't prolonged up until the point where it becomes pointless drama. Ichikawa Kei never disappoints.