Alice, My Friend Wants to Stay at My House...In My Bed...With his Girlfriend -- Ask Alice: Advice for Life at Home

Hi Alice, I have a friend, who was my flatmate, but now lives with his family. His girlfriend lives in a different city and now they want to meet up with each other and stay over at my apartment in my room, asking me to sleep on the couch outside for a night. He's a good friend of mine, but I do not want him to stay in my apartment and in my bed with his girlfriend. How can I say no in a way that does not make him feel bad? Thanks, Hesitant Host