Well, the stuff that glues all relationships together (romantic and platonic) is of utmost importance, but I'm quite in favor of sex and think it "literally embodies" the divine.
My only caution is to not get too caught up in the hope/expectation of a "perfectly-balanced triad/delta/triangle." People are all so different from each other, that each relationship is bound to be different as well, and you can't always expect each person to be "equally attracted" to both of the other persons. It's important to let each relationship be what it needs to be, and value the diversity of those relationships. One relationship might be more emotional, another more physical, who knows.
I'm not saying don't have that fantasy because it's good and a rather sweet idea. Just remember that like in any good story, the fantasy will morph and evolve to fit the ever-changing/unpredictable curve balls that life always throws our way.
I understand the feeling of having something missing in your life, and I hope you and your husband will both find the perfect dream person for you.
Love means never having to say, "Put down that meat cleaver!"