"For... some kind of very specific spell?"

"To know if you're likely to get pregnant, if you want a baby?"

"...oh, right, some people want babies. I forgot."

"I didn't know what a clit was, and you forgot babies existed: we're a matched set!"

"Well, I knew they existed! The most important feature of sex in the Scholomance is making sure they don't happen!"

"Which I guess you could achieve by only having sex with people of the same sex."

"Exactly, si-mei!"

"--That makes the conversations make more sense. I guess."

"Okay, so guys can do analogous things to the things girls can do, with hands and mouths and grinding on each other and stuff, but they can also have anal sex." He points to the prostate. "The prostate is inside a guy but when it's stimulated it feels really good and the easiest way to stimulate it is by putting fingers or a dick or a dildo inside his ass. --A dildo is a fake dick made out of metal or plastic or something. --Anal sex can also feel good for women but they don't have a prostate, there's just a lot of nerves around your butt." 

"You do kind of need nerves there-- I'm getting distracted."

"Judging from Nie Huaisang's oversharing they did things with hands and mouths and also anal sex."

"--Yeah that conversation is making a lot more sense now."

"But si-mei! You should give yourself orgasms or you're going to be full of horniness you haven't dealt with and then the second a guy gets you alone in your room and starts flirting with you and touching you and talking about sex, you'll get so horny you won't be able to think straight and then you're at high risk of babies," he says, obliviously, a guy alone in Chu Chu's room and flirting with her and touching her and talking about sex.

Chu Chu is much less oblivious about that. "I'm not too worried about that?" Seeing as so far they have managed to stay carefully at the diagram and hypotheticals stage here. They're getting an A+ on 'not doing sex things that will get them killed' so far, even if they're grades are slipping in 'not ending up in a situation where that's a concern.'

"Also it's fun and you should have fun things."

"I appreciate the thought?" That should be an appropriate response. Hopefully.

"Also you definitely can get pregnant the first time you have sex, birth control potions don't work, if he tells you he has an abortion pill he's lying, he's definitely not going to pull out of you before he orgasms, you shouldn't let him put it in you for a second just to feel it, and whether or not you'd let him do it if you loved him if he loved you he wouldn't ask you to."

"I have absolutely no intention of doing anything that could possibly get me pregnant."

"--and if someone tries to make you tell us and we'll kill him."

"...thank...you?"

"Or you can kill him yourself. That's probably better, all things considered. But if you can't manage it."

"I will keep that in mind. Thank you." She'd rather not kill anyone, regardless of what they have done, but-- that's probably not a good idea to advertise here?

"Anyway. The reason that people masturbate and sleep with people of the same sex-- or, like, one of the reasons, people like me would do it anyway because it's fun, but the reasonable responsible people like Ayako do-- is that if you don't the horniness kind of builds up and then you make terrible decisions? And if you're taking care of it proactively then that's not going to happen."

Well that's an alarming prospect. She's not sure how likely it is, but-- Wei Wuxian knows more about this stuff than she does, so maybe. "Would you notice the terrible decisions?"

"Well, yeah, but you'd kinda be too horny to care until afterward."