are you automatically the secondary if you are dating one person in an open relationship?
It really just depends on the nature of the relationship. If they are in an established relationship, and have shared goals, like common bills, common assets, raising children together, etc., and you don't share in those responsibilities, then yes, that would be a "secondary" role as most folks define it. You may not be labeled as such, but that term is useful is setting out the nature of your relationship with them.
Now, if you get to the point where you're living with them, sharing in the financial and child-rearing responsibilities, etc., you might become a "co-primary".
Try not to get hung up on the labels, though. In many relationships, a secondary is an extremely important part of the relationship, and in many cases, even if they don't share in the household responsibilities, may be "equals" of the heart in terms of the love that is shared.