I understand your issue about the need for secrecy. Many times it is a necessary evil if relationships with parents could be destroyed, a job could be lost or there are young children in the picture that are too young to deal effectively with the consequences. In both of my poly relationships (one was a v in which all lived together for a couple of decades until our v passed away), I was the one who was added to a marriage - and hence kept secret. (I prefer not to use the term secondary, because in no way was I secondary in importance.)
So here are some of the questions I asked myself in regard to the same issue you are now having:
1. Would telling people about the relationship increase the love I share with this couple? The answer for me was that it would not.
2. Would telling people about the relationship make it more likely to last or be successful? Again, my answer was no.
3. Would telling people about the relationship boost my self-esteem, preventing people from viewing me as a lonely loser? Ding-ding-ding-ding! So I'd uncovered my first reason for wanting to tell the world - I wanted to incorporate my relationship into my self-identity, as in now my self-identity says, I am a successful, desireable person because I am involved in this fantastic relationship. Armed with these facts, I decided to incorporate these into my self-identity WITHOUT the need for external validation by the society at large.
Now as to the security issues, first of all marriage is a false sense security. Marriages end in divorce all of the time. I expect this feeling that marriage creates security actually goes back to the whole self-identity thing.
While this is all new for you all, the thing of it is if your relationship gets to the point where you become a permanent fixture in their lives, there are things that can be done to increase actual security like buying a house together that is all of your names.
Since I am not clear on how new all of this is, whether you live with them, etc., I am giving generalities. I hope you find these helpful.