You can get the sex scene as long as you asked for a Relationship advice from Saya, be it on Route A (chose "Keep pushing" and convinced Sho to see his mother) or B (chose "Let it go"). And for that day specifically the sex scene was the best outcome.
I just think out of 3 outcomes of Day 26, "Let it go" was the worst one. Shoichi is just overworking himself and lashes out on others (like in the scene with Kiga on Day 31), while Yuuichi is too scared to say or do anything about it for a while.