It's not about about my, or anyone's, expectations.
It's about how the scene faded to black while they were kissing on a couch and then they woke up the following morning sleeping together, blushing (and with Chiyo not wearing her pants/skirt). Plus in a later chapter they were going to do it in Igarashi's room back home too until they got a cellphone call. Based on that of course most people would think that they already had sex, since that's what the storytelling was implying.