I think I'm going to agree with CielDuMatin.
Whilst the actual wording is to be considered, the much more overriding factor is context and delivery.
Calling someone your primary/secondary makes for a quick and easy way to convey your set up with two people. Most poly people know that a primary is usually someone you live with/are committed to in more ways than just dating (living arrangements, marriage, fluid bonded etc.) whilst a secondary is usually someone you are only dating.
Using the terms in a degrading, hierarchical way, however, is pretty awful. As has been quoted already "You're only a secondary" is, to me, a sentence that will get someone dumped faster than a sack of lead bricks.