This is straight up a story about a guy getting attacked by a succubus that he can't confirm is an actual succubus. The decision to not allow any dramatic irony and revel in the ambiguity is brilliant, and I'm fully convinced this is intentional.
The number and intensity of the dreams are unnatural, the girl gives a vague reason for having sex with him, and she evades the question every time the guy asks for the truth in both reality and dreams. Her look and silence on the second panel on page 13 is the best place I can point to. Everything about their relationship is both intense and ephemeral, and that combination is what is eating the guy up and prevents both him and the audience knowing for sure.
Ooh, a sequel to Dreamy Romantic? Nice. Certainly wasn't expecting one, but I'm glad to see it.
Well hey, if she does catch your cold, then it'd be your turn to go visit her, no?
At this point I'm pretty sure it's all real, so I'm a bit puzzled as to why he's so unsure whether it was a dream or not. I guess if he keeps passing out after sex, it might feel like it was a dream...
Yoshiki-kun: "I wonder if this will lead to some sort of Okaa-San Inquisition."
Okaa-San (suddenly bursts onto scene accompanied by jarring musical chord): "NOBODY ESCAPES THE OKAA-SAN INQUISITION!"
But Seriously, it's quite possible that his mother *koff-koff* "suspected" something might happen between him and Kurosu (if it didn't happen already), so she hastily departed for work, leaving them (a-hem) alone together.
The number and intensity of the dreams are unnatural, the girl gives a vague reason for having sex with him, and she evades the question every time the guy asks for the truth in both reality and dreams. Her look and silence on the second panel on page 13 is the best place I can point to. Everything about their relationship is both intense and ephemeral, and that combination is what is eating the guy up and prevents both him and the audience knowing for sure.