<p>Various animal species form pair bonds, typically defined as affective and specific relationships between two adult individuals. While pair bonding often leads to notable physiological and behavioral changes, the specific mechanisms underlying the formation and maintenance of these bonds require further in-depth investigation. In this study, we explored spatial and sociosexual behavior in eight mixed-sex dyads of common marmosets (<i>Callithrix jacchus</i>) across repeated short-term exposures before and after pair formation. Proximity between initially unfamiliar individuals increased with the duration of visual exposure, but after pair formation, subjects showed no proximity bias toward their partner compared with other conspecifics. Sociosexual behaviors, including male penile erections, female tongue protrusions, and lip-smacking in both sexes, were more frequent before pair formation, when subjects were still housed with their natal groups, than after the transfer to pair housing. Interestingly, after pair formation, these behaviors were more frequent between less familiar conspecifics than co-housed partners. These findings suggest that while increased interindividual proximity may reflect habituation between initially unfamiliar individuals, it is not a reliable marker of established pair bonds in this species. Likewise, the sociosexual behaviors analyzed here appear to reflect initial courtship and pair formation rather than bond maintenance. The persistence of sexual interest toward extra-pair individuals supports the view of a flexible mating system in common marmosets, indicating that controlled exposure to outgroup conspecifics may provide opportunities for the expression of species-typical sociosexual behavior in captivity.</p>