Some aspects of mating behaviors shift in response to external context, which may be mediated in part by hormonal mechanisms. We examined the influence of population density on mating behavior of the hermaphroditic nudibranch, Berghia stephanieae. Individuals were reared communally (high density) or with one partner (low density). We video-recorded mating between test pairs and, using tracking software to quantify movement, constructed behavioral ethograms of courtship sequences. We found that when a test pair included an individual from a low-density environment mating duration decreased, with post-copulatory mate guarding often omitted. We also found that test pairs exhibiting similar, intermediate crawling speeds were more likely to mate. Fast or slow individuals often did not interact, perhaps indicating they were not in a state conducive for mating (e.g. startled, hungry). It is known from other species that temporary internal states can have strong impacts on mating behavior and that shifts in hormone levels can occur in response to social environment and induce behavioral change. We therefore tested the effect of conopressin, homologous to mammalian oxytocin/vasopressin. Conopressin, when bath-applied at nanomolar concentration, decreased latency to mating. However, unlike the effect of low-density rearing, conopressin did not affect mating duration or sequence. Likely a cascade of internal mechanisms, more complex than a single hormone, is responsible for the effect of rearing. In summary, we found that prior social environment altered mating dramatically, changing both the sequence and dynamics of the behavior, whereas conopressin had only a transient effect on latency rather than overall characteristics of courtship.