hiii! wait... it's so cute you thought of asking me 🥹 yeah, i'm happy to share my two cents!! a fic that describes & visualises sexual content (in most cases) gets an M rating. i think of it as the 18+ tag. in your case, your fic doesn't contain sexual content bc fade-to-black smut isn't actually smut, so T could be appropriate, BUT i think it actually depends on whether you ever cross the line between suggesting to describing sexual feelings/actions? i'd probably ask myself this:

do you describe lust/nudity in detail? do you mention any sexual terms (e.g. blowjobs, prepping, fingering, groping etc)? do you include sensual descriptions of the body, how it feels, dirty thoughts? is their makeout sexually charged (ie. do they purposefully feel each other up), and do you describe arousal, or a strong awareness of it?

if your answer to any of those questions is yes, i'd rate your fic M. if you're merely suggesting, and all of your language is oblique and implying something relationship-defining is about to happen... a T rating works fine.

you probably already have an intuition about whether your fic is a T or M. don't overthink it! for me, personally, i just rate it higher when unsure. like, remember goggled man? that whole fic was just about flirty fun at the pool, which is sooo rated T, but i rated it M just for one paragraph where soobin's lizard brain reminisced about yeonjun blindfolded on his knees asking to suck him off on camera, later whispering in his ear that their editors would never know. do you sense the difference there? imo that's what M sounds like. it isn't thaaat explicit, but the sexual content warrants an M tag.

when in doubt, just leave an author's note at the start :) and thank you for liking my stories, it means so much to me 💖 is this your first fic...? congrats!!!