What people opt to do in their sexual relationships is up to them but moving your three way partner into your home and having your children call her mom is sorts of fucked up. But people prioitise their sexual desires above their children in all sorts of situations.
But if you believe the literature that being polymourous is all about your ability to love, commit and maintain relationships with more than one person, it isn't that much of step futher I suppose.
The problem is if you are monogamous then you don't actually believe that you can love, be intimate and provide in a relationship at the same level if you are also in a romantic and sexual relationship with someone else. Because you can't. All things will never be equal.
So you need to be able to accept what ever that person is willing to give or not give to you versus the other partner.
And what does he actually mean? Is their version of secondary relationships out side their main relationship? Have they engaged in three way relationships before?
Do they have rules in place that have to be followed to maintain the primary relationship? That you are expected to fit into?
Personally for all the rhetoric about the ability to commit to one person and how no one person can fulfil all your needs and desires it actually shows the opposite. They aren't capable of commitment. Also its incredibly unhealthy to expect people to fulfill you. You should be able to do that yourself.
It's all very enlighten in its positioning v monogamy and yet as you consider it further and in any depth, its all centred around you 'working through' ie suppressing normal feelings in relation to relationships to give into the wants of the person you are sleeping with.
Honestly the thought of my husband going out to meet his 'girlfriend' fucking her then coming back to our bed and being intimate with me makes my skin crawl.
It's one of the worst aspects of being cheated without the knowledge that was happening.
I would be really challenging yourself on if you are even considering this because you haven't actually dealt with the trauma of being cheated on. And this option provides a false sense of security from it happening to you again. It doesn't. He has as much ability to betray your trust and boundaries as any one else.