And the she later tried to hook him up with the big titty author. Maybe Sasha just didn't like this woman.
I’d say a big part is that Yumenomi was for research. Any interactions were for the sake of improving his art, something that bonds them together. Also she explicitly approved intercourse there whereas here she only expected a reading while the girl went on and teased him and felt up his hand. And anyway he didn’t get far with Yumenomi before she put a stop to it so it’s hard to tell how well she really would have handled it.
One big thing to me is that all of this, now and in said later interaction, she is setting boundaries and giving approval for what can and can’t be done, something she’d previously declared to be the privilege of a lover only. If you’re dating you have every right to decide these things but otherwise you have no say. By setting boundaries like this, even subconsciously, she is entering territory that in her heart is restricted to girlfriends. She’s his girlfriend in her heart even if it’s not official.