Well, that's the thing, yes, OP answered his own question with the reason that their previous girlfriend left them, was to pursue a primary relationship of her own. I think that when deep feelings of love exist, one would wish to either escalate the relationship, or have at least a open and family connected thing...and if she had a child, maybe she wanted a male influence in her kid's life, too. Thing is, it's just part of relating that sooner or later, many people will develop a desire for "more" than just good times.
So my poly experience was with a quad (me, boyfriend, and a married couple) and I also had my separate relationship with Zen off to one side of that, not involved really with the quad, though they were acquainted. While I loved the quad, and I still do, and I have no big complaints and there wasn't an explosion of drama, near as I could tell, or any huge awful reason it had to end...I did end my involvement with them as girlfriend. Why? Because it did not feel REAL. It felt like I went on vacation to visit their world every weekend. I would never be a resident in their lives, just a tourist.
Being on vacation is fun! Being a tourist is fun! But it isn't the real, deep stuff that ultimately I felt a relationship needed to be made of. And I chose my mono-guy and parted with my quad, because in my couple with Zen there was room to grow. (Also: the very legit fact that I was simply stretched wayyyy too thin on time & energy, trying to juggle four relationships, parent a teenager, work, etc.)
In what you propose, even if it's really fun and really good, it just feels like there is a sort of glass ceiling, it can only go so far and then you bang your head against the couple's rules or shall we say...the way things just are.
Does that mean it's this terrible idea, doomed to horrible failure, that should never be attempted? NO. I don't think so. I am very deeply glad I had the year with my quad, and the experience of loving them. It was glorious and I'll treasure those memories. But I never expected it to last "forever" (I have a hard time expecting that of most things, but I feel a year was a good run with the quad, whereas I wouldn't bat an eye thinking my relationship with Zen could go for decades.) So if you're ok with living in the now with any unicorn you might find, and them moving on if they need to...you moving on if you need to...and especially if you're skilled at amicable partings, as opposed to drama bombs...then go for it! Have a good time!
Now I have to speak to "one penis policy" a little. So Zen and I are closed. But. He has mentioned that he feels uncomfortable with me being in a relationship with another man, but would not feel comfortable with me in a relationship with another woman. I can accept this. Frankly he fulfills my male-partner-needs with a thoroughness that makes me not want another man in my life, so I don't argue with what your wife tells you, OP. But I sometimes think...I'd love to have a female BFF, who is down for getting together, movies, hiking, snuggles, even sex. That would be pretty neat.
Here's why I'm not pursuing one. I don't want to be expected to be part of a threesome for her and any man in her life. And I don't want there to be any expectation for her to join Zen and I, if he doesn't interest her. She has a right to like or want whatever she does. I'd want it to be ok with everyone if only she and I had a pairing and no one else was IN it. But I think that any males involved would be wishing they could get some of that girl-girl action. Both of us might be wishing, even secretly, that we could give that to our men. And if I found an outright lesbian, she might not be comfortable that I have a male partner and also like men. So.... There is so much potential awkwardness. I'm like, nah, ya know, I'm ok.