[Coldsaturn]

Yes it is heavily implied that they have sex in his birthday. But if you think about the wedding banner, Caleb is still talking about wanting more from their status quo like they're still negotiating their relationship. The line he says in the cafè when he has his wedding outfit on is about wanting to be every role in her life, embracing the gege role that he was so conflicted about in the past. So my idea is that birthday-wedding-beach happen one after the other and in close proximity, so they 100% know about their reciprocal feelings, have also had sex etc, but they're still navigating the logistics of what it means for them to go from siblings/childhood friends to this. There's a kind of awkwardness (especially from MC) like she sometimes forgets that the boundaries have changed between them.
Caleb's suggestive lines to me confirm that we're after the first time they've had sex. They're still awkward about it, but Caleb sounds to me very much like "I wouldn't mind doing that again like now. Just saying"

Hell yeah to the horny on main thread. What are the rules? Do they need to be in public?

I'm still debating whether evol hijacking is retconning previously established lore. I think I remember Raf (and then Sylus obviously) implying that resonating needs two willing participants. But this banner seems to imply that we don't need the person's explicit consent in that moment, only a generic positive attitude toward MC that allows the resonance, and then she can guide it on her own.
Or maybe this is a level up in power for MC compared to the beginning of the story, which would make sense.