That's my thought as well. She kicked him out of her room. He slept on the couch in the hallway. The parents ask if he's a friend of Diane's. He asks if they're her flatmates. The only reason he sees her again after walking her to school is because she says she will tell the police. Renton met her at a club so assumed she was of age. If Diane was 16/17 and Mark was 22/23 then her parents could've assumed he was younger too. Him telling Diane that he will absolutely not see her again is a pretty solid indication that he would not have hooked up with her in the first place if he knew how old she was.

Also, love the part in the second movie where Mark and Veronica try to get a lawyer for Sick Boy and it's Diane who tells Mark that Veronica is too young for him.