广 is a building open on one side; 木 is the wood inside it. The oldest 床 wasn't only for sleeping — it was a low platform for sitting, dining, and receiving guests. Beds were ancient China's sofas, desks, and thrones combined.
Li Bai's 床前明月光 'moonlight before my bed' is the first poem every Chinese child memorizes — though scholars still argue whether his 床 meant bed, stool, or well-railing. 起床 'rise from bed' starts every Chinese morning routine and every boarding-school bugle call.
Wood 木 under a roof 广 — the BED you don't want to leave.
婚床 the marriage bed gets scattered with dates, peanuts, longans and lotus seeds — a rebus wishing 早生贵子 'bear a child soon'.