Draco's having a hard time coping with the world post-war, and he uses sex and alcohol to get by. It never occurred to him that Hermione might be coping in a similar fashion.
People cope in different ways. Most settle for drugs and alcohol, and Harry's was sure that his next therapy patient couldn't be much worse than the addict he'd seen last week.
Until Draco Malfoy walks through his door with a type of addiction he'd never worked with before.