For me, a lot of it has to with information and control. The need for control triggers the need for information.
When my husband is dating, seeking out a new partner, we talk about this a LOT. Whenever he comes home from a date, we talk, he shares all his feelings, and unless he becomes wrapped in NRE and forgets about me for a bit, I am in the center of his attention and he reassures me and because he comes home after every date, we can check in, we can reconnect, and I don't feel threatened.
My lover MrBrown has 2 other partners, and he dates every now and then, and has occasional hook ups that I know nothing about. Because I know almost nothing about his daily life, and because we don't communicate much in between dates, I have no information and I don't feel the need to control and I am not threatened. Sometimes he will tell me about his other relationships or dates when we see each other, but I'm sure there's lots he doesn't tell me. I'm perfectly fine with that.
My lover C is dating and looking for another relationship. We are in a very vague and undefined place when it comes to how much we share and how much we tell each other. Sometimes we communicate daily about everything. I feel i want to share, and know what he's doing, who he meets, how his love life is developing. He wants to share, but he also wants to be free. He doesn't want to tell me everything. And that is where my fear kicks in, and there's this gap between the control I want to have and the control i can have, and I'm getting mixed messages.
I'm working towards becoming less controlling... and being able to enjoy the amount of sharing we do, without it meaning that we have to share everything. But yeah, with him? I'm one of those established poly people with 3 partners who occasionally has an anxiety attack when one of those partners is developing another relationship.
Cleo - forties straight female
Ren - husband of 20 years
Bo - BF of 1+ year
Lou & Jane - Ren's girlfriends
Lin - Bo's other GF