I have never heard this one, so I can't place where it comes from, but going by the OP's name, it is East Asian. It is a myth; there is no reason not to eat them at the same time, or for that matter, any two fruits at the same time. read more
No, for most people, it's not harmful to eat watermelons and peaches together. Some people find that the taste and texture don't go well together, and a few random unsupported web pages suggest it's not good to eat anything with watermelon because the water would slow down digestion of other foods. read more
Applying honey on watermelon to make it more sweet is also dangerous in fact eating honey before or after eating watermelon is also equally dangerous. Don’t eat eggs with yoghurt; The combination of pickle with yoghurt is also dangerous. Eating fruits like watermelons and bananas with egg is also dangerous. Eggs do not form a healthy combination with beans, fish, milk and meat either. The combination of cherries with milk is extremely dangerous for the body. It can even cause the death. read more