Yukino has no solid reason to answer him positively--MC-kun has been cheating almost constantly for a while now, with exactly the people he's saying Yukino is "getting along" with (at least, I think it's those two he's referring to; one of them expressed almost the same sentiment earlier). This, or he's been signaling a flagrant refusal to commit to her alone. That said, I doubt the author will write Yukino's awareness of that--there are only so few chapters remaining.
Speaking of those two, the taller shorthair is one thing--at least she tried to reverse when she thought about NTRing the girl she claims is her friend (again), but the shorter one is far worse. She stood there next to Yukino and told her, with a wide and enthusiastic smile plastered on her face, that she's just so happy to have so much fun with all of them--including Yukino, the same girl she's fully intent on cucking with full cognizance thereof. That's not just shameless, it's psychopathic.