18+ Hajimete no Sex Friend

My first "friends-with-benefits" was also the first person I ever fell in love with.

Kiyotaka Sano and Tamaki Aihara—two people with basically zero romantic experience—match by happenstance on a dating app and end up becoming FWB. They both fall for each other. But they each convince themselves that if they don't keep acting like "calm, casual adults," the other will toss them aside. So even as they keep sleeping together... they can't seem to let their hearts get any closer...

(Source: Shogakukan, translated)