Green’s Dictionary of Slang

swinging n.2

[swing v. (8g)]

[1960s+] indulging in husband-and-wife swapping parties; thus closed swinging, swapping parties in which only husband-and-wife partnerships take part, no singles are allowed to unbalance the situation; as opposed to open swinging, in which all-comers – married or single – are welcome and all end up in the same bed.

In phrases

soft swing(ing) (n.)

[1970s+] a partner-swapping party where the only intercourse is performed by couples who arrived together; non-penetrative sex, however, is enjoyed at random; thus soft swinger n.