The reality is that any change of any type effects any relationship.
Therefore, a new relationship is effected (regarding how it comes to be and how it operates) by the existence of existing relationships.
This is EASY to see and we ALL know it's true when we look at the manner in which romances are carried out for someone with children, versus someone without. There are songs about how different it is for "a single mom" for example. The lyrics from Brad Paisley's songs "He didn't have to be" are a great start for that one.
"When a single mom goes out on a date with somebody new
It always winds up feeling more like a job interview
My momma used to wonder if she'd ever meet someone
Who wouldn't find out about me and then turn around and run
I met the man I call my dad when I was five years old
He took my mom out to a movie and for once I got to go
A few months later I remember lying there in bed
I overheard him pop the question and prayed that she'd say yes"
It's not the same when no children are involved, because there doesn't need to be any concern regarding the existing relationship needs AND RESPONSIBILITIES that don't exist.
The same is true in a poly dynamic. If you have pre-existing relationships ESPECIALLY live-in relationships, then new relationships ARE going to alter them. It doesn't matter if it's new friendships, new babies, new siblings, new parents, new lovers etc. There will be effects on your time/availability/attention/focus/responsibilities etc. That's just a facet of life. To ignore that, is to be more than naive; it would be irresponsible.
HOW MUCH it's effected depends greatly upon the type of relationship you already have existing and the type of relationship you are starting.
If your current relationships are tightly entwined-there will be more impact then if they are more loosely entwined.
If the new relationship is more tightly entwined-there will be more impact then if it is more loosely entwined.
So, for me, my life is TIGHTLY entwined with my family. This isn't just my 2 loves. It's my two loves and my 2 children at home, 3 (and 1 spouse) children not at home, 2 grandsons, 3 of my 6 siblings (and their spouses) and a network of friends who aren't "just friends", they are family. I'm talking daily interaction with all of them. I'm talking several times a week of social interaction with all of them.
Plus my schooling and the home schooling of the 2 kids at home.
So for anyone "new" to date me. They are going to suddenly be part of a large, already established family network. Some people cringe at that idea (which is ok-but means we aren't going to date). This IS the life I have. It's been the life I have. My chosen family includes every single person I have ever dated, because breaking up doesn't mean that they aren't part of the family still.
ALL of these people are welcome in our home "without an invitation" and they all randomly appear. Our home is very much the "safe home" for a network of people somewhere over 50 people strong. There are only 5 of us living here ALL of the time. But on any given day any or all of the rest could appear.
This is "normal" for our life.
On the other hand, my sister is living with her boyfriend and his wife and their 2 youngest children. They never have company over (including family) except the maternal grandmother who babysits often. They also almost never socialize outside of their group as a family. My sister is regularly here, she is also a huge part of our greater network. But her boyfriend, his wife and the kids-not at all. The wife and the boyfriend have dated other people-and never met each others partners. They don't bring them home. The kids certainly aren't involved at all. It is interesting to note-that their marriage has been on the rocks for years and they fight endlessly over the "you did this" and "you did that" bs because the segregation that they keep-means that if either of them wants to see someone else, it results in them not taking care of some function at home with the kids our the house in order to be gone.
They've had issues with my sister, for the same reason when she didn't live there. She struggled because she was lucky to get a day or 2 every 8 weeks with him. Now that she's living there she "see's" him daily just as the rest of their family does. But she still struggles with not getting any time for romance (which he also doesn't get with his wife enough for anyone to be happy) because of the ongoing struggle with "who is responsible for what and whom" arguments.
That isn't true of everyone.
But, I think that having a more segregated poly style is functionally only possible in scenarios where there isn't as much overlap of responsibility. Situations that are more independently created (like Marcus describes) where each individual has their own individual responsibilities and there aren't a lot of joint responsibilities. The more responsibilities you have, the less time you have. The more joint responsibilities you have the higher the chances of someone getting resentful if you aren't there holding up your end of the bargain.
I know for me; I'm not going to be the "live in nanny/housekeeper/cook" to our household so that the guys can be gone for days at a time with someone else.
Someone else can come here.
Likewise-we JUST YESTERDAY had it OUT over the same subject-not regarding poly.
I'm not going to do it for hteir jobs either. Maca has been gone for work 6 of every 8 weeks since Mid May. The job won't finish until after Christmas.
I told him point blank. NO WAY NO HOW will that happen again.
I don't give a shit if it means losing his job. I'm not going to tolerate it. We have a joint responsiblity. I didn't get married so I could be a single parent (and yes GG is here-but he's been working 60-80 hours a week as well).
I also told GG-no. No way. No How. Not playing it.
The kids are emotionally devastated. It's not fair to them. It's not fair to me. A job requiring more than 40 hrs a week on a regularly basis. NO. A job that takes you out of town for more than 2 of every 4 weeks. No.
The same goes for another partner.
If I am going to have the responsiblilites of a single parent. I'm going to have my OWN home. My OWN life and not be held to the requirement of workign "with" their needs and their requests.
(which I don't want and neither do they-but it bears repeating since their bosses seem to think it's perfectly reasonable since they have me here to "pick up the slack". But I'm not the "slack picker upper".)
"Love As Thou Wilt"