Any time you add a new Player it will mean adjustment. This is true of new child in the family as well as new lovers.
When its a bew child, we dont allow ourselves the mental escape option of kicking them back out. So we are forced to address how to learn to accept them and what changes in the family must be made for them to be a part of it.
This can seem easy-but it can be quite difficult too-consider a special needs hild whose needs take large quantities of time and attention from the other siblings.
Assuming there are no red flag behavior issues:
I think that if the issue is addressed from the perspective of I have to find a way to Accept that there IS a new person in the family it becomes easier to do. So long as kicking them out is an option, that seems easier and more "worth it".
Much like the idea that if you consider divorce a viable option there's less motivation to resolve the complication in a relationship.
Obviously-this is all arbitrary in case of abuse issues. Any issues of abuse must be addressed separately from the topic of "can i or cant i handle poly"
"Love As Thou Wilt"