Re: Were there any people having open relationships in Trek?
Phlox on Enterprise is probably the closest example. He has 3 wives, who each have 3 husbands, who in turn have 3 wives... apparently into infinity, and says he has several dozen possible partners in this arrangement. And he seems to consider Ensign Cutler's suggestion of an extramarital relationship.
Curly: Moe, Larry, the cheese! Moe, Larry, the cheese! (Horses Collars, 1935)