This whole primary/secondary might be easier to secure when everyone involved is relatively young and "unattached".
To me, having kids with person/people A instead of B pretty much shows you consider them your primary/primaries. Although I plan living with/close to Moonlight & co at some point, kiddies are definitely out of the discussion. Part of it is of course the fact that they have kids already, and I wouldn't want to create "competition" in a way. And the greater part is that because I consider Vanilla my primary, kiddies will enter into discussions with her.
But the love languages might help. If hers is quality time, indeed pointing out when he's made plans to hang out with her instead of you might work. Or if it's physical touch, then ensuring that whenever he passes her by, there is a embrace, a caress or a quick kiss involved. Words of affirmation maybe don't work for her so much, because I understand that he has told her already multiple times how he considers her his primary?
Me: bi female in my twenties