It is widely acknowledged that in most species sexual selection continues after mating. Although it is generally accepted that females play an important role in generating paternity biases (i.e., cryptic female choice), we lack a quantitative understanding of the relative importance of female-controlled processes in influencing variance in male reproductive fitness. Here we address this question experimentally using the guppy Poecilia reticulata, a polyandrous fish in which pre- and postcopulatory sexual selection jointly determine male reproductive fitness. We used a paired design to quantify patterns of paternity for pairs of rival males across two mating contexts, one in which the female retained full control over double (natural) matings and one where sperm from the same two males were artificially inseminated into the female. We then compared the relative paternity share for a given pair of males across both contexts, enabling us to test the key prediction that patterns of paternity will depend on the extent to which females retain behavioural control over matings. As predicted, we found stronger paternity biases when females retained full control over mating compared to when artificial insemination was used. Concomitantly, we show that the opportunity for postcopulatory sexual selection (standardised variance in male reproductive success) was greater when females retained control over double matings compared to when artificial insemination was used. Finally, we show that the paternity success of individual males exhibited higher repeatability across successive brood cycles when females retained behavioural control of matings compared to when AI was used. Collectively, these findings underscore the critical role that females play in determining the outcome of sexual selection and to our knowledge provide the first experimental evidence that behaviourally moderated components of cryptic female choice increase the opportunity for sexual selection.