With all due respect to Ishtar...
If the person you are involved with maintains the concept of primary and secondary relationships in the way he has structured his version of poly, he may not see his relationships as equal or 'matching up'. It's hierarchy based and one of the relationships is a higher priority in his life than any subsequent relationships that he might add. Such hierarchy can be based on any number of things. It can be time/length of involvement, marital status, different types of relationships that a person has, or a combination of these or other reasons.
Right about now is where people have the knee-jerk butt hurt reaction because some folks interpret hearing 'not as important as the primary relationship' to the equivalent to 'not important at all'. It doesn't work like that. It's not like if it's not 50/50 (time, attention, etc) that the only alternative is 100/0. Currently, my split is something around 80/20 as far as independent time, which works for everyone involved. (I'm counting that as 'just' relationship time with either of them independently. Not co-mingled time or time that is set aside just for myself, friends, or other endeavors.) It works well for us.
I never said anything about the new relationship being "equal" to the primary (or original) relationship.
Everything I said still holds true whether you're ranking your relationships hierarchically, or whether you consider them equal.
Even in a situation where the original relationship is granted a special, or a higher, status than the new one, and where the new relationship is expected to defer to the old one in all ways, there is still going to be an impact on the old relationship by adding a new partner into the mix.
- If time is spent with the new partner, there is less time available for the original partners to do things together.
- If activities are done with the new partner (even if the new partner pays for their own share) there is still an impact on the budget of the old relationship.
- If the new partner moves in, there is still and impact on household arrangements.
- Even if the new partner doesn't move in, but merely spends time at the old relationship's domicile, that impacts everybody in the household, merely because a new person is present during meals, tv watching, cleaning activities, recreational activities, etc, etc.
- If the old partners interact with the new partner on an emotional basis, this is going to impact their mood (they'll be happy at times, have arguments/fights at times, feel tired from spending time with them, feel more energized from having played with them, etc, etc). This impact the new person has on the emotional state of the people involved with them is going to impact the old relationship.
And so, on and so on.
Whether that impact the new person has is big or small, depends on a whole bunch of factors, including how good the new person 'fits' into the old relationship, how much contact there is with them, how much the old relationship is given priority instead of compromises being made, etc, etc.
But the impact is going to be there.
Just like there would be an impact if you develop a new friendship with somebody, and start spending time, energy, and resources, on developing that friendship with that new person.
To think that you can have a new relationship in a vacuum, where the interaction with the new person doesn't impact the original relationship at all is ludicrous. Any new social interaction you have with a new person, or even with a new hobby, is going to have some degree of impact on all previously established relationships and hobbies, even if it's just a change in your mood as a result.