In chapter 34 they tried THE SEX before, so it's not surprising.
With that said on chapter 34, Fake Miyu and Yuushin also speak about this "Real Yuushin" and that is repeated here with this Real Yuushin talk. What are you really hiding you bastard sex-haver?
This concept has been around since chapter 1. Yuushin is characterized as someone who has a hard time showing his real feelings and being anything but "kind" thanks to OG Miyu's influence in his childhood, to the point he willingly accepted being bullied. Chapter 7 and 8 showed both him and GF Miyu are aware of this, it's not a shady secret or anything and that the more he hangs around her, the more his real self comes out.
He's very likely not hiding anything considering we get to see his thoughts all the time, unless they pull some amnesia or DID thing with no foreshadowing. The story didn't even hide GF Miyu originally suspected him.
What I'm more curious about is OG Miyu flashback saying "I heard [Yuu] just arrived at the station" to Haruto, considered Yuu's flashback showed the only message he sent OG Miyu was "I can't go, sorry" from the hospital bed after SHE was the one who told him she was at the station.