Not really serious and utterly tongue firmly in cheek but:
"You want me to pick you up some lube at the store? The rate your going to rub the skin off it"
"Change hands after 20 or you'll give yourself carpal tunnel syndrome"
"God I jerked off to better material than this at your age"
"She's faking it and that's clearly another guy's cock"
"that's got to be a deepfake, you can see the artifacts"
"you got enough Kleenex?"
"If you don't want to be doing your own laundry by hand at least take your shirt off so its not got spunk stains on it"
"I'd better not find any socks as hard as a hammer head"
Or if sexting "You need better lighting" "put some effort into composition" "at least tidy up behind you so it doesn't look like your posting from a dumpster" (for son 18+) "I hope your at least charging them" "you know there's a lot of guys who would pay good money to watch you do that, work smart not hard"
I guarantee they will suddenly decide they are no longer in the mood, I knew someone who wanted to do phone sex, her mother was aghast until she saw the pay rates and suggested they do it together, cue sudden loss of interest entirely and the words "ewwwww gross"
Teenage daughter
"You know you'll give yourself an infection using that" "At least put a condom on it" "If your going to use something as a dildo, just ask me to order it rather than risk an er visit to get broken bits removed" "There's nothing wrong with masturbation, but use the proper tools for the job, if you want a vibrator look one up and i'll order it, just remember to clean it so you don't get an infection" "Are you ok, I thought from the noises you'd fallen and broken something, next time play some music or even message me to say your taking some "me" time so I don't come in thinking you need bones reset or your otherwise injured"
Sexting: "your sending THAT gargoyle pictures, have some standards for heavens sake, he/she/they could give satan a scare in the dark", "If your going to send "racy" photos at least brush your hair so you don't look like you just crawled out of a hedge" "If your going for hot...well underwear that even kinda matches is a good start and NOT ones with holes in them, have some standards" "You know makeup isn't meant to be applied with a plastering trowel, nor do many people find the pennywise clown look hot either" "You know if you want to attract someone, you need to dangle some eye candy for them instead of dressing like a nun or an old lady, when I was your age if my skirts did more than barely cover they were too long, why are you such a prude?" (I know someone whose mother gave them this speech, they were utterly mortified lol)
Etc....
Sometimes an accepting "helpful" parent is far worse.....lol