A couple of months ago, SW and I talked about how our casual relationship had evolved into something more. Yet we will never be each other's primary. Although we did acknowledge to each other that if our loves had not been in the picture, well, things might be different, he's committed to his girlfriend and I am to Beloved. I don't know if SW would characterize us as a secondary relationship - we haven't talked about it - but that is the role that relationship fits into for me. Right now, the primary/secondary/casual model works pretty well right now for myself and Beloved and I suspect it works for SW and his girlfriend.
I've been thinking about a couple of issues lately that might be of interest to the OP. First, I'm not a particularly envious person. (I took one of those online 7 deadly sins test and scored high to medium on all the sins, except envy. This cracks me up.) However, I am competitive. So when SW posts that he is spending time with his favorite girl, I get a twinge of but "I want to be the favorite!". Then I get a grip, remind myself that I can't be 'number one' in this case and move on. (I'm also an only child and so very used to being 'number one'.) It's not a problem for me or for SW and I's relationship but I am curious how others handle not envy or jealousy but competitiveness.
Second, I wonder how others build intimacy outside of sex with secondary partners. I've noticed that I feel most intimate with SW during and after sex. Of course, sex was our reason to get together in the first place so that's not so surprising. However, we have lots else in common besides boinking. But I don't feel as intimate with him doing other activities, even though I always enjoy hanging out with him. We tend not to talk about difficult or intimate topics except after sex. I wonder if this is a 'dude' pattern. The women I've dated created intimacy throughout the relationship and not just in sex. I'm going to talk to him about it but wonder, again, if others have experienced something similar and what they did about it.