Yukari's casual outfit went missing so she had you look for it because she had something to do, so when she left, you went to Makoto's room to convince him to help you search for it. But when you entered his room you saw him wearing the outfit, so you obviously took a picture to blackmail him like the good friend you are.

(Everyone is 18+)

Original image (by sieroo):

https://rule34.xxx/index.php?page=post&s=view&id=9879863&tags=makoto_yuki+crossdressing