The above conversation about "primary" and "secondary" relationships was interesting to read. It really gets me thinking!
Although I am not legally married, my partner and I have been an open couple for roughly 13 years. We've both had other not-exactly platonic relationships in the mean time, but not many (very few!) and none lasting long. I live with my partner, Kevin, as well -- and so I suppose we are in some sense "primary" by some folks way of evaluating such matters. But I feel as if I am capable of having two (three at the very most!) partners with a similar level of commitment, involvement, love, etc. I'm pretty sure Kevin is also capable of that, and we could do that whether or not we're all involved with each "romantically" or whether or not we're all living together, regardless.
If I had another true love, he or she would be a full true love, not a partial love that would lead to me wanting to think of him or her as a "secondary". That said, I think it would be possible to have a partner, aside from Kevin, who doesn't want the same level of involvement and commitment as I share with Kevin. But I simply don't find myself interested in casual sex -- or sex without some sort or degree of emotional bonding, tenderness, warmth, affection... and even commitment.
But "commitiment" needn't mean "forever". It has to be commitment to loving, intimacy, honesty, vulnerability.... So it can't be primarily about sex. I've had my fill of being used as a sex toy.