The origin of sexual reproduction remains uncertain. Prevailing explanations for its emergence focus on the evolutionary advantages of genetic recombination, particularly the elimination of deleterious mutations and adaptation to changing environments. The proposal presented in this work addresses a different question: how reproduction, once autonomous, became dependent upon interactions between distinct organisms. It is argued that sexual reproduction originated from an ancestral host–parasite relationship. From this perspective, the sperm cell is interpreted as an endosymbiont and fertilization as a process homologous to a parasitic infection. This hypothesis provides a possible mechanism for the transition from individual reproduction to reproduction based on the union of two organisms and implies a profound reconsideration of the origins and nature of sexuality.