Simon really likes kissing. Specifically he really likes kissing Grace and the noises it makes.
Back on Eden romance wasn't really something he'd gotten the chance to experience. Sure he's kissed people before, but it was more of a quick press of lips together before jumping away to make sure they hadn't been caught. Intimacy was frowned upon unless it was related to reproduction, and Filament Station happened before he'd really had the chance to even think about something like that. Besides, it's not like he was able to interact with anyone much during his sentence anyway, so why bother worrying about it.
The first time Grace kisses Simon it's very different to what he'd been used to. It's not a blink and miss it brush. No, Grace keeps his mouth on Simon's and actually presses their lips together.
Simon is about to lean in and press back because it feels amazing, but then he realizes he's being kissed in front of Rocky! Simon startles away from Grace so quickly he almost trips, and when he sees the hurt look on his face he stamers out some incoherent gibberish while gesturing to their audience.
Grace's face relaxes as he seems to understand what just happened, but then turns annoyed as Rocky thanks Simon for stopping and rants at Grace for making such disgusting noises in front of him! They start snarking back and forth about annoying things the other does, and Simon slowly realizes there's no one to get in actual trouble with for such an act, not anymore.
As the conversation moves on to something else, he gets more and more flustered that Grace kissed him. Grace kissed him! And only stopped because Simon pulled away! Why did he do that! He wants Grace to kiss him more! So later he corners Grace while Rocky has disappeared to eat or something (he wasn't paying attention) and asks for more kisses.
He hears a surprised delighted laugh and then there are hands holding his face and he's being kissed again like earlier, only this time Grace hums into it. Simon's knees go a little weak because of it so he raises his hand to rest on Grace's waist for balance and feels a smile against his mouth.
He feels like he's floating. Kissing someone has never felt better and each time their mouths part with a quiet wet noise, he floats a little higher.
He's not sure how long they stand there gently kissing, but Rocky literally has to roll his ball into them to get their attention. Simon's just glad they were standing so close because he does nearly fall but Grace is able to hold him up so he can get his footing righted.
Rocky goes on another rant about how disgusting human mouth noises are and Simon is the one to argue back this time, saying that no the noises are really really nice, actually! Grace gets a little more red in the face (he was already a bit flushed from kissing for so long) and partially hides in his hands while watching the other two argue
