LIKE the family in the cartoon below, more and more middle-class parents, especially in the cities, are beginning to be confronted by a rather unusual problem: unmarried teen-age lovers who want to share a bedroom on visits home. Some reactions:
> Atlanta public relations director:
"I tell my daughter and her boy friend, 'When you're a guest in my home, we try to make you comfortable. I would appreciate it if you would try to make us comfortable by not sleeping together when you're here.' "
> Wife of a Manhattan investment banker: "My daughter has a very aggressive boy friend who's against hypocrisy...