1). I think that when you enter into a new relationship with someone, there are generally going to be things more important to them than you are. It could be a career, a relative, a friend, a hobby, or a lover. They may be open to prioritizing you over these things eventually, or they may not. When speaking to me, if someone says, "I can't be do x with or for you because of y," I don't think it's different if y is a career, a marriage or a hobby. I think there is a tendency to focus on other relationships because of some (to me unrealistic) idea that all relationships should be equal and a tendency to view metamours as competition. Sometimes one relationship gets priority over others for a number of reasons.
For my purposes, let's say couple privilege is prioritizing one relationship over another.
I think society gives couples privileges triads, quads etc do not have as thatgirlingray already mentioned
2. The way I have most often seen it unfold is this. Single person x starts dating partnered person y. Y says they are always going to prioritize primary partner c. Even though x is uncomfortable with this and understands the potential consequences, they choose to date y anyway. At some point, y has both c and x asking for some of the same resources. Y usually gives them to c. For some reason, x is surprised by this and expects y to change. I don't understand why this is so common.
3. I think it's beneficial for everyone to set the priorities in their own lives and be clear about them. Though I sometimes wish people's priorities were different, I think it is best that everyone decide how to divvy up his or her resources based on his or her priorities. I don't think there is any "correct" set of priorities.
4. I may have been affected negatively by couple privilege when someone i dated for years dumped all of his partners (including me) because he met a monogamous person he could see himself marrying one day. My understanding was that we were working toward a primary relationship. He said that his relationship with this person was more important than all of his other relationships. It was painful for me, but I think he had to invest himself in what he thought was most important. In the end, I'm not sure if it is really a case of couple privilege or simply a case of being disappointed that he had not been honest with me about the importance of our relationship.
In another case though, I have been equally affected by a partner prioritizing a career over me. That was just as painful, though it was more difficult for me to (temporarily, in the heat of my frustration) see a bank account or title as being responsible for the pain in my relationship, while on the above case it was easy to use the new partner as a scape goat to avoid my disappointment in my partner.
5. Consenting adults should enter relationships under any condition they like. I have had plenty of people proposition me with arrangements I had no interest in, but I told them why that wasn't compatible with my desire and moved on. I think it's a bad idea to try to tell everyone which agreements they should make en mass, though I think it can be helpful to give your opinion to people who are interested in it.
6. I am part of a partnered couple. I tell people it is unrealistic to expect the same commitment from me that I give my husband before years and years together. My husband got to where he is by years of trustworthy, caring, compassionate and reliable behavior. He earned his prominent role in my life. I think it would be wonderful to develop something that deep with someone else, but I'd expect it would take years.
If my husband told me it was monogamy or divorce, and we could not find any other way to resolve the issue, I would be monogamous. I made specific commitments to him first, and I will not consciously break those. If someone one day reaches that level of commitment, they will be entitled to the same promises from me, and my husband has agreed to promise the new person equal rights in every way should that time come. Until then, he does have priority. My other partners know this, and they have consented to pursue a relationship with me under those terms. I don't believe the terms are "right" or "wrong". We're all consenting adults making informed choices.
7. My approach to poly is different than some in that I am only willing to date people who understand that most of our time together will be spent in groups. They can bring one or more partners or friends with them. Affection between any combination of people is acceptable in these groups. Most of the time, in these groups, my husband doesn't get a very large portion of my attention because I am more focused on people I see less. At bedtime, it's usual for people to split up in unplanned ways. If a partner wants more sex or wants a private conversation, they are absolutely free to ask for it, and I will accommodate them to the best of my ability. Anyone who doesn't want this is free to pursue a relationship that fits their desires.
I have never been in a situation where my husband didn't want me to date someone I was interested in or vice versa, so that issue has been nonexistent. Our plan in such a case is to identify the reasons for the desire and work on those issues.
The issues my husband had with my other relationships and vice versa were easy to solve by everyone involved talking it out in the same room or over a conference call.