Disclaimer: I don't own Percy Jackson and The Olympians; Rick Riordan does.
It starts with "No birth control is foolproof. I'm pretty sure you and most of our friends are proof of that. And we're way too young to have a baby." It continues, a couple of years and a cross-country move later, with "I'm sorry, but I'm still not ready." Finally, one night in the apartment they share in New Rome, in that liminal time between going to bed and falling asleep, Annabeth rolls over to face Percy and whispers, "I've thought about it a lot, and I think I'm like my mom."
Percy times his fingers in Annabeth's blond hair, his nails lightly grazing her scalp, and says, "I know you're like your mom. I'm not that stupid."
"No, I mean in terms of sex."
"I'm not following."
Annabeth laughs lightly. "Of course not, Seaweed Brain." She stretches forward to kiss Percy before continuing. "You know the reason behind the running gag about children of Athena not having belly buttons, right?"
"Oh yeah! You're born of a meeting of the minds, right? Not by sex?"
"Exactly," says Annabeth, stretching to kiss Percy again. "My mom is one of the Three Virgin Goddesses. She doesn't have sex. I know they didn't have this term in Ancient Greece, but I think she's asexual. And I think I am too."
Percy's quiet for a while, and then he says, "What does this mean for us?"
Annabeth is quiet for longer before responding. "I'm pretty sure I'll always love you, Seaweed Brain. But there will probably never be sex."
Percy kisses Annabeth's forehead. "You like what we've been doing recently?"
"Cuddling and working out and not having to save the world every five minutes? Yeah, it's been lovely," Annabeth says. "Why?"
"Because I like it too," Percy says emphatically. "I don't need more than this. I have you, and a home that's safe. That's all I could ever need. There are other ways to deal with what my body wants."
Annabeth nuzzles her nose against Percy's cheek. "I really, really love you, Seaweed Brain."
"Were you actually worried about talking to me about that?" Percy asks, caressing Annabeth's cheek.
"I mean, kind of?" says Annabeth. "We do come from a family that has a lot of sex. Like, an almost world-wrecking amount of sex."
"Yeah, but . . . Annabeth. I literally remembered your name and nothing else when I got my memory wiped. My love for you was already that strong. And think about how much we've been through since then. We've literally trekked through hell together. You know this is forever at this point, right?"
"I mean, yeah . . . But no sex? You're sure that's okay?"
"Pretty sure my horniest years are behind me, to be honest," says Percy. "And I made it through my teens all right without sex. So, yeah. Literally all I need is you and some more time off from saving the world."
"Like forever?"
"Yes. Both of those. Forever."