For the record, I never read any gendered imbalance into any of this. And I think that a LOT of what you've described and where you've gone in your thinking is right on target. But it's very, very easy to end up acting in controlling, unfair, and hurtful ways to a third person entering a relationship with a long-established couple without ever meaning to in the scenario you've described. As someone who's in a not-dissimilar situation to what you're looking for myself (see my signature, or my blog -- my gf and I don't consider each other "wives", but that's because our relationship is still fairly young, and we're taking it slow, exploring where we're going... could happen some day!), I've become sensitive to those issues. Reading other people's horror stories is really depressing. :/
I hope I'm not projecting onto you by trying to tease out any potential issues. As I alluded above, we've seen some real horror stories on here from both sides of the unicorn/unicorn-hunters scenario. Where I got concerned was, indeed, about the sexual exclusivity issue, but not because I think there's anything wrong with polyfidelity or exclusivity generally, not at all. Mono relationships, closed poly, I'm all for it. What works for you is what works for you in that regard. Rather, it raised a red flag to me that something you guys never needed before -- exclusivity -- should be a non-negotiable need for the new person. I mean, how is that *not* supposed to look a little odd? I see that you're requiring the new person to give something you didn't give each other and I ask "why"?
Because you two both were involved with the other couple and it's harder for three people to all be involved with the same folks? Because you were comfortable with your old friends but the idea of the new person bringing up an interest in unknown people is too uncomfortable? I hope and assume it's about those things, which would be a lot healthier than just a need for control. But for you to say "our marriage involved sex with other people before but if our new spouse wanted something like that with someone they liked, rather than with the people we like, no way jose, we'd drop Mr./Mrs. Third like a hot potato if he/she wanted that"... I mean, surely you can see why that might make someone say "woah, what?"
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.