What if you never find someone who loves you? What if you find your true love, but they get struck by lightning? What if you fall in love with the Doctor and know that he'll never ever love you back the way you love him? Living in a world of "what ifs" is only going to drive you mad
Instead of worrying out and planning out your poly future, why don't you come up with some standards of what you need in a relationship, and go along and enjoy the ones that meet those standards?
For example, whether my current/future partners are single, married, or dating, I want the potential to be a full part of their life-- at least with them and their family and trusted friends. It's not always possible to be out in the open about being poly. I want them to be honest, to have similar values, and similar ambitions. But I don't concern myself with the details of how the structure will lay out.
By the way, you don't have to "settle" for secondary status, merely because your partner is married. You can, if you prefer limited obligations in relationships, but I, and others in this forum, have relationships with married partners that are not limited in any way (at least not as concerns love, affection, and importance).