It is typically assumed that sexual selection unilaterally drives the evolution of reproductive isolation, but selection for increased reproductive isolation could feed back on sexual selection when these processes share a genetic basis. Direct selection for isolation is most likely to occur in the context of "reinforcement", where selection acts to increase pre-zygotic barriers to reduce the costs of heterospecific matings. Most studies of reinforcement focus on pre-mating barriers to reproduction, however post-mating traits, such as conspecific gamete precedence, are also ubiquitous barriers to reproduction that can potentially respond to reinforcing selection. Conspecific sperm precedence (CSP) also has a known shared genetic basis with intrapopulation sperm competition, allowing for the possibility that selection for increased CSP in sympatry could alter intrapopulation sperm competition specifically in these sympatric populations. We test this prediction with the sister species Drosophila pseudoobscura and D. persimilis, using two sympatric and two allopatric populations of D. pseudoobscura. Consistent with a pattern of reinforcement, the sympatric populations had higher mean CSP. Reinforcement, in turn, constrained sexual selection in sympatric populations by decreasing the average offensive sperm competitive ability within populations, allowing less opportunity for sexual selection to operate. These data demonstrate that strong reinforcing selection for reproductive isolation can have consequences for sexual selection and sexual interactions, in these important postmating sperm competition traits.