I would say that what matters most is the context. How are you portraying emotion in that scene? Is it just physical sensations and lust, or is there intimacy, emotional connection? This is just my opinion but personally if a sex acene is just physical description and doesn’t show us anything about the characters other than their sexual activity, or if it doesn’t directly move the plot forward and could be done with a fade to black scene, then it shouldn’t even be there. Unless you are writing erotica, because that’s where itbbelongs. Sexy times for the pure sake of titillating the reader basically belong in erotica. Could you tell the story without the physical description of the sex acene? If so, does it need to be there then?
For example, JEHallows has a trilogy of fanfiction where there are quite a few steamy scenes between the protagonists, but she fades to black after the foreplay, because the detailed scene isn’t necessary to move the plot forward. She conveys the passion between the characters without needing a graphic description. There are also sex scenes described like poems,that describe sighs, caresses, emotions but not any actual physical description of body parts. This style isn’t to everyone’s tastes and some readrrs want detailed sex, but I wouldn say, why not read erotica then? I mean, I stopped reading the Anita Blake vampire hunter book when the sex became too often and far too needless.
I don’t see a problem with fade to black, it’s not childish and silly if you do it well. It’s not boring.
Another point about context. It’s really really important to portray sex healthily, especially in a teen book. No romanticised abuse. I think it’s important to portray sex as positively healthy and loving in a teen book, to send the right message. Or to describe toxic, unhealthy sex in a clear way that doesn’t romanticise it or see it as positive.