Swinging having been described as "definitely CNM", I'll mention some other folks I know where there are others in their sex lives, but only for the sex.

Another couple I know (hetero couple, but I'm not actually sure how they would define their own orientations) have a very kinky sex life and routinely take part in sex acts with participants outside the relationship (for example, two or more people together tying up / dominating a third). They've been together for something approaching three years now (and co-habiting for over a year), talk openly and happily about their sex life, and in general seem happy and committed to each other.