Hey there. You've stumbled into a lot of negativity that is likely because what you guys are considering -- "bringing in" a younger woman to your established relationship (there was actually a thread devoted entirely to people's distaste for this type of language, due to it's correlation with problematic situations: http://www.polyamory.com/forum/showthread.php?t=22905
), moving her in right away, expecting exclusivity from her as a condition of the relationship -- are all hallmarks of the sorts of poly relationships that tend to be the most explosive, messy, painful, etc. It's so much, so fast, when things are still so new, so volatile, and frankly so imbalanced. Can it work? Maybe! Does it usually? No. And the person who gets burned worst is typically the newer, almost always younger partner.
There is a huge power imbalance between a long-established couple and a younger single person engaging in a new relationship with one or both members of said couple (great essay on navigating this situation: www.morethantwo.com/coupledating.html
). By moving in she will become dependent on you. If/when it becomes messy, she may well lose her place to live. If/when she wants to be with someone else, she will lose the relationship (why is it that your husband gets to decide he wants to become involved with someone new, but she never gets that freedom?). If/when she decides she ever wants kids, she will lose the relationship. How is any of this fair to her or teaching her good things about relationships?
If he wants to date her, fine, good, there is nothing wrong with that. He seems to have kind of a paternalistic attitude towards her (what makes him qualified to teach her what a relationship should be... isn't he a flawed human being like the rest of us? doesn't she have her own things to give, to teach? and who's to say his love will protect her, when it might well break her heart worse than the situation she's in now?) but ok, an older man/younger woman dynamic can in theory be healthy. But don't assume you can incorporate her into your lives after a few visits and have it go well.
It takes a lot for two people to blend their lives, a lot of sharing and compromise. Are you willing to give her all the same opportunities to mold her surroundings, to arrange the house to suit her, to ask for sacrifices and compromises from him, the way you and he have surely done for each other in the course of your relationship? Are you willing to let her be a full partner in devising a new threeway partnership... or are you just willing to let her find a niche in the dynamic you've already created? If you cannot in good conscience say that you can give her full partnership -- and honestly, with all of this being theoretical at this point you can't promise anything, as you don.t know how you'll actually react when things are reality rather than imaginings -- then don't put her in the difficult position of living with you early on, and don't be so selfish as to demand exclusivity. Let her live her own life as an independent person and he can date her as such, visiting her in her own place, letting her live her own life with him as a part of it. Why on earth not do it that way?