When a young child begins to ask questions about where babies come from, adults are likely to search for simple, innocent ways to explain the idea of reproduction. With young children, detailed descriptions of the penis and the vagina are excessive, so adults may resort to describing how the bees pollinate flowers to make new plants. Or they may point to how mommy and daddy birds make nests together and take care of baby birds together.
Those simple, childish examples eventually came to symbolize the first sex-talk that little children receive.