I am rather biased in the area of discretion. From the point of view of the secondary, if the couple is demanding secrecy it can become unhealthy and even harmful for you. I was in a relationship that demanded I keep it secret from my friends, our mutual friends, their family etc. Some secrecy can be good/necessary but it went a lot farther than that. And, one day, when a mutual friend found out, he broke up with me and broke my heart saying that I had betrayed him. Now, I'm not saying it would necessarily go like that but making it such a high stakes thing is asking for trouble some times.
I found that the friends who did find out were pretty ok with it. They might not have understood fully or maybe even liked it but they were supportive and there were no direct negative consequences in those friendship because they knew. I don't think you have to come out right away but as a relationship gets more serious it's gonna come up sooner or later. You can always start out by introducing the person as your friend and gradually taking more folks into confidence about it. Sometimes it helps to start with one person who you really trust who will be likely to react positively. Just be aware that who ever you get involved with deserves some recognition of what they mean to you. Don't hide them away like your dirty little secret. Keep in mind, that that secrecy can strip anyone, esp. a single secondary, of their much needed support system. I would say, only keep it as secret as it needs to be.
I remember I compared my poly relationship when explaining it to a friend to being gay. I said, it's a different way of doing things just like if I told you I was gay. They thought about it for a bit and said, oh, ok. Overall, my friends reacted supportively to my poly explorations. If you have lots of pagan and gay friends, there's probably a good chance that they won't be very judgmental about it. What are you most worried about when it comes to telling people?