Your daughter's boyfriend is staying over the night. They've been together for four months. She's 18 in a month's time, the responsible and mature type. Dates rarely. The boyfriend's a little younger than her, shy and polite and a good kid all around. She says they "haven't done it" yet, and you have every reason to believe her - very good parents/child relationship here. The family is middle-high class, fairly secular with a modern outlook, in a mid-sized California town. Where would the boyfriend sleep?
ETA: the story is set in 2003. The relationship is as stable and calm as it gets.
ETA 2: the good parent-child relationship part also means that they're going to communicate expectations and concerns to each other, such as - assuming the parents are so inclined - asking if she'd be comfortable with their sharing a room or. You may assume a large enough bed. The parents and the boyfriend are getting along. There's a 10-years-old sister in the house.
ETA 3: thank you, I think I have more than enough information! Thanks you all for your feedback - I learned everything I needed to know and more, and this ended up being a very interesting discussion.