Mate swinging is a topic researched by researchers who prefer functionalist perspective. According to them, mate swinging helps sustain sentimental bonds of marriage by providing participants an opportunity to release their sexual fantasies for an evening or weekend.
Classical studies:
Walshak, M. L. 1971. “The emergence of middle-class deviance subcultures: the case of swingers”. Social Problems 18: 488-495
Denfield, D. and M. Gordon 1970. ‘'The sociology of mate swapping”: Or the family that swing together clings together” Journal of Sex Research 6: 85-100