This is one I go back and forth on. I am involved with a married couple and so consider myself a second even though I have never been made to feel second. Sir has never been overly enthused by the word "second" either but I don't really see any way around it.
I tend to have some issues because in many ways I see them as one because they are married and so I am the one who makes myself feel secondary.
As to your OP, that is something else we have been dealing with. I have been in this relationship for only a year. There are a few different dynamics in our relationship besides the power exchange. For one, I am really the only open poly one. Sir is open to the possibility but extremely picky and pretty busy with the two of us. S (wife), she is more poly-fi when it comes to emotive relationships.
So far other than talking/flirting with someone I thought might be a prospective partner I have seen no one else since becoming involved in this relationship. I am very leary about looking for other partners at this point in time because Sir doesn't stick with a set of parameters under which he would be ok with my becoming involved in an emotive relationship with someone else. For him it seems to be that it depends on the person, situation etc. I don't feel that I can get close to another without having a more stable set of expectations and knowing what is okay with the both of them. I need for stuff to happen naturally. In return, he needs to be in control.
We talk often about this and have worked through quite a bit in the last year but I am still scared and uncertain about getting emotionally involved with another. So, I completely understand what you mean only from the flip side.