I feel like one of the seldom mentioned ideas in the poly community is relationship bleed over. That is, the idea that what we do and feel with one partner can bleed over into relationships with others. No matter how hard people try to keep things separate, inevitably feelings will go through. For those who need/want things separate, I imagine this can be difficult – part of why I feel like DADT relationships often fail in the long run.
Sometimes this manifests as something as simple as RRE (renewed relationship energy), where the NRE from one relationship helps reinvigorate other, older relationships, sometimes even to the point of being NRE-like. Or happiness from a special celebration (or just a nice date), affects how you behave or what you talk about on other dates, or during time with other partners. To use a more physical example – the kind of sex you have with one partner in the morning, can affect what kind of sex you want with another in the afternoon or evening; even with some people, whether you’re physically able to have it.
This is a two-sided sword. RRE can be amazing, but if you’re the partner in the older relationship it might feel like compensation. Compersion is amazeballs, but if it starts to feel like an obligatory feeling, then that can suck majorly. And if you were really looking forward to doing X, Y, and/or Z in bed, but they did W, Y, and Z earlier and are not up to (or able to) do it now, that can be quite a disappointment.
Where do I stand on this? When I have amazing metamours, this tends to be less of an issue and more of an occasional bonus. Even then, if something else is going on (whether at work, home, in other relationships, or in my head) it can affect how much bleed over is awesome and how much it can be annoying. I love when my partners have things going awesomely in other relationships – it makes me very happy. But sometimes, especially if our time together is limited, I’d really like to talk or otherwise connect as us and more than the usual amount of bleed over can interfere. And I try to respect that in all my relationships.