Male and female unisexual flowers evolved from hermaphroditic ancestors, and control of flower sex is useful for plant breeding. We isolated a female-to-male sex transition mutant in melon and identified the causal gene as the carpel identity gene <i>CRABS CLAW (CRC)</i>. We show that the master regulator of sex determination in cucurbits, the transcription factor <i>WIP1</i> whose expression orchestrates male flower development, recruits the corepressor TOPLESS to the <i>CRC</i> promoter to suppress its expression through histone deacetylation. Impairing TOPLESS-WIP1 physical interaction leads to <i>CRC</i> expression, carpel determination, and consequently the expression of the stamina inhibitor, the aminocyclopropane-1-carboxylic acid synthase 7 (<i>CmACS7</i>), leading to female flower development. Our findings suggest that sex genes evolved to interfere with flower meristematic function, leading to unisexual flower development.