No way would I want to sleep in the same bed as both my parents! I would sleep in a bed with my mother - but not both my mom and dad. Especially as an 18 year old girl! If there was not an extra room or pull out, then I'd ask for some extra blankets or something and make a bed on the floor.
OTOH, if I knew a couple were coming to stay at my house with their 18 year old daughter I would try and find someplace else to put her apart from her parents - kids need their own space as much as the parents do. I wouldn't think of putting her in the same room if at all possible, and if it was not possible, let them know before they got there what the sleeping arrangements would be and if it was a problem for them they might feel like they'd have to make alternate arrangements.
I've slept in the same hotel room and camper as my parents, but as an 18 year old girl - I'd rather sleep on the floor than in the same bed as both of my parents! But maybe I'm just an uptight Canadian girl!
BB