Every relationship is unique. It would be tough for me to be monogamous because of my curiosity and love of watching things unfold and grow, not to mention those damned sagi tendencies (not that i hold any credence to astrology, but its fun to use as a rationalization).
That said, a real relationship, which I assume are the relationships you ask about, takes time and energy*, but does anything else fun take anything less? Anyway, I have neither in big supply. It is not practical for me to have multiple relationships right now. I mean I could, but they would be shoddy, unreflective, and inattentive (This doesn't jive well with my capri tendencies). Maybe this is an over-romanticized view of BDSM, where Doms have serious responsibility over their submissives and their well-being, but if I am going to do something, I want to do it well.
*For "secret" relationships, multiply these factors by 5.3. Honesty definitely reduces load and subsequent friction.