It is pretty much within MC's character, it's one of the approach I like in this VN, not everything needs to be said out loud. The VN uses the visual medium to do its storytelling.
If it's in character largely depends on players' choices. In my playthrough, I kept as much distance from FMC as possible.
In that case, how can it be in character when he's only just lost his virginity, and never showed interest in another woman? Yet suddenly he turns into a horndog who can't keep it in his pants. Until then, he's portrayed as someone who's too hung up on Yukine, never even considering other girls/women. (Except Hikari, as you mention)
What happened with Hikari wasn't unfaithful, it happened before Yukine returned. When Yukine returns, MC is thinking a relationship with Hikari is wrong, so he's not cheating on Hikari either. I do get the argument you're making though, that falling for Hikari is a sign of the MC opening up to women and relationships. My counterargument is that Hikari wore him down, and took a lot of initiative to get to that point. She also has the benefit that the MC loved her since she was born.
I don't think the harem was on MC's mind on that sofa. Even when confronting FMC in the bathroom later, he claims he felt lonely. It wasn't an experiment, just a moment of vulnerability that FMC exploited. She starts by humming a lullaby that's special to the MC. She also claims multiple times that she knows him through and through. So either she didn't know him well enough to realize a lullaby that his mother used to sing, wouldn't cheer him up, or she chose that lullaby on purpose to make the MC even more emotional.
What you said "about needing to be close to someone", can be easily observed. We can say MC feels lonely and vulnerable, with Akemi creating distance, Hikari running away and Yukine not communicating like she's supposed to which got MC worried and think that a Harem is viable option. And also what we see is MC using FMC to fill that hole that they left, arguably unhealthy but well within reason and his character.
Why would the MC think the harem is a viable option, when Yukine (at that moment, still his one true love, depending on the player's choices) is ignoring him? If anything, that should discourage him from risking the relationship even more. More so because in chapter 6, Yukine doesn't know anything yet. Yet it is MC who initiates intimacy by asking "does your offer still stand" (harking back to the shower scene, where I rejected FMC's advances).
In a playthrough where the MC is immediately pursuing FMC, I'd agree entirely with you.
Being unable to reject FMC in the sofa scene makes a lot of sense in the story if you start rejecting FMC after that, I believe it's a much needed scenario. Plus the sexual tension between FMC and MC has been really high at that point.
Yes, I think we more or less agree on that. I wrote "the story has to go on", with that in mind. And I really appreciate that they did have that talk in the bathroom. But what I wrote about the sofa, were my thoughts at the time the scene played. MC's behavior was jarring to me, since up till then, I was under the impression that my choices were actually building up to something. Instead, he grabs the boobs of someone he hasn't really shown interest in, and cheats on the love of his life. (I realize I'm repeating myself a lot. I'm sorry.)
Right now I'm pausing the game, since I ran into a bug. I didn't tell FMC about the vision, yet at the start of Chapter 15 I'm in her bed getting a BJ. I'll have to dig through the code to see how to correct that (according to the walkthrough this scene shouldn't play).
