Yikes, yeah, I think the thing with the tags is out of line. I'm sorry that you feel so attacked, and I can definitely see why you do. These boards are supposed to be here to help people but if we just drive someone away when we see something that sends up an alarm we're not exactly helping.
To answer your original question, yes, it's completely normal to feel uneasy about the idea of your life partner having children, which are a lifelong commitment, with someone you barely know. That should be something that is not decided now but left as a possibility for the three if you to work out if things ever got that serious.
Like I said in my post above, I think your situation reminds people of a lot of other really sad, painful stories that have passed through these boards and it's those similarities they're reacting to as much as the particulars of your situation. Not exactly fair to you, and I think it could have been handled more sensitively, but on the other hand if there's a dangerous pattern that you're stumbling into it, you ought to be aware of it.
Your language is, I believe, a big part of how you've been received. I linked to that one thread on "joining/bringing in" and then there's wording like "We aren't looking to continue to add and so would release C from us" which makes it sound like a) you and he have already decided the rules of this new relationship, based on what's going to work for you, specifically something that maintains your current dynamic and gives you guys freedoms (he gets two partners) that she doesn't get (she may only ever have him unless she wants to get dumped), and b) like you don't take her seriously as a potential partner (released? that sounds so benign, even pleasant... but getting dumped because you want equality isn't nice at all).
It's clear that you want what's best for C, you are being open-minded to what many would see as a radical idea, and you are taking the time to think about it. That's all really good. I hope that despite your negative experience here, if you happen to be reading this, you can glean something useful from this thread. Please understand that you can want what's best for someone and yet still, completely unintentionally, be unfair and hurtful to them.
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.