Drosophila melanogaster seminal fluid proteins (SFPs) secreted by the male reproductive tract are transferred to females during mating and induce profound female post-mating behavioral and physiological shifts. Many SFPs work in concert with female-derived molecules to promote fertility. However, reproductive fluids are a battleground for evolutionary conflicts such as male-female sexual conflict, sperm competition, and cryptic female choice. These forces have driven rapid diversification of reproductive molecules across species, with lineage-specific gains, losses, and gene duplications shaping the complement of reproductive fluids. Despite this rapid turnover, conservation of functional classes is frequently seen, suggesting evolutionary constraint on fundamental reproductive processes. One such functional class of SFPs is the family of odorant binding proteins (Obps), which are common members of the arthropod ejaculate. Work in Drosophila has shown Obps are highly abundant in chemosensory tissues and mediate behavioral responses to odorants, leading to the hypothesis that seminal Obps act as molecular carriers for ejaculate-derived anti-aphrodisiac pheromones or molecules important for fertility. Using functional genetics, we found that Obp56g is necessary for male fertility and induction of the post-mating response, due to its involvement in mating plug formation and sperm storage. Evolutionarily, we found extensive lability in copy number and evidence of positive selection for a subset of seminal Obps. We found that Obp56g appears to have gained expression in the male reproductive tract over evolutionary time, suggesting it has been co-opted for a reproductive function. Proteases and protease inhibitors are another major class of SFPs, which likely interact with female-derived proteases to mediate reproductively important proteolytic cascades. Work in Drosophila has shown that one route of SFP gene birth is co-option via duplication, where an ancestrally female-expressed gene duplicates and subsequently evolves male-specific expression. We found evidence of this mechanism for the male SFP CG17242, which likely duplicated from a cluster of female-specific proteases. We found evidence of duplication, pseudogenization, and rapid evolution for the female protease cluster, while the male SFP is under purifying selection. Together, these results demonstrate commonalities and differences in the evolution of male and female reproductive genes, adding to our understanding of sexual coevolution and conflict.