Ahhh, I love the merpeople love language!!! Now I can't get the image of Luke teaching Aemond how to kiss out of my head, carefully avoiding the other's sharp teeth. It also makes me question, does Aegon know what kissing is? Cause he knows some things about humans. I could see him go all in, shove his tongue down Jace's throat (maybe his understanding of kissing is pretty vague) while also rubbing his feet against him. I'm so in love with this AU!
Aw, yes, I was also thinking about that. 🥰 This headcanon got long...
Precisely because of the sharp teeth, merpeople only do closed-lipped kisses. Even like that, kissing on the lips is considered kinky. They don't have sharp teeth when they're in human form, but they rarely get intimate in that shape - Aemond never - so French kissing is simply not a thing.
So, when Luke leans in for their first kiss, Aemond turns a fraction away so that it lands on the corner of his lips.
Luke pulls back to frown at him only to find him completely flustered (by Aemond standards of course). He leans in again and presses a kiss to Aemond's cheek this time, and Aemond just melts into him. Let's him kiss all over his face, his eye closed, his hands holding Luke tight and his feet caressing Luke's. Who knew that chaste kisses could be so sensual?
Days later, when Luke eventually manages to explain that humans kiss on the lips, Aemond is wary about trying it. It doesn't sound appealing to him. But then it happens, and - Luke's lips are so soft. They feel plush and plump as they press against his, and then there's tongue, and Aemond is scared. He doesn’t want to hurt Luke, not even with his not-so-sharp human teeth, but Luke whispers words he by now understands, It's okay, so he lets Luke part his lips.
For the longest time, he just holds his mouth open, careful not to move so that Luke doesn’t cut his tongue on his teeth. When Luke draws back and laughs, he wants to call the whole thing off even though it felt good. Deliciously sinful. But then Luke tells him to copy him. That it's not going to hurt. And Aemond is so weak for that temptation that he does it, licking cautiously at the seam of Luke's lips and then his hot mouth, and it's explosive. He has never felt this turned on before. His whole body is burning, and he's making a sound no decent mer should but it seems like he left all his decency behind. He doesn't think he'll ever be able to give this up.
Aegon knows what kissing is. Real kissing, not that sorry excuse his people do, he thinks. He's not as uptight as his crab of a brother, he likes to experiment. He spied on human couples having sex on the beach multiple times. So, when Jace closes his eyes for that first kiss, about to lean in, Aegon has full confidence in what he's doing. Sure, it's his first try, but he knows humans have soft teeth so he's not nervous at all. Goes for it with relish. Okay, it's a bit weird to stick his tongue down someone's throat, but he kind of likes the choked-off sound Jace makes. Jace tastes pretty sweet too. Yeah, Aegon has human kissing down perfect, thank you.
Side note: you know what else merpeople don't do with their mouths, right?