I dont know about watermelons but there are a plethora of plants that can not produce fruit without being pollinated by another plant. Also if you ever self pollinate a plant you'll have to recognize the 2 different parts. Is it just the calling them male/female that bothers you?
Edit: I guess I should say plants/flowers can have a sex, fruits I don't think would. They are just seed dispersers I believe.
Hmm I'm no plant expert, so maybe I'm wrong about this but I thought fruit always grows after pollen moves to an egg like part of the flower/plant, so the 'sex' of the fruit is always a combined pollen+egg like cell. This cell develops into a seed while the surrounding plant grows the fruit for various reasons. Maybe there is a heterozygous genetic trait in some plants where we could label the individual as sex A or B, but I thought self pollinating plants were basically both sexes at the same time??? Idk... Maybe I should do some googling but heck the fediverse needs content :P
The scientific definition of "fruit" is the ripened ovary of a flowering plant. This differs from the normal usage so some things not commonly considered fruit, such as tomatoes and the pods of soybeans, are fruits by this definition. Flowering plants (not all plants have flowers) have male and female anatomical structures. Many species have both structures in one flower. Some species have flowers that contains either male or female structures. These flowers can either be on the same plant (monoecious), like watermelon and corn, or on different plants (diecious), like papaya. The ovary, what will become the fruit, is a female anatomical structure, and it makes no sense to talk about a male fruit for any type of flower. Male flowers produce pollen, which fertilizes the embryo in an ovary, but male flowers themselves don't produce fruit.