What is wrong with polyamory? [I Support Polyamory]

Just finished an argument with another user saying our kids need to be taken from us solely because we're polyamorous. Their only argument, it's a sin. Couldn't even explain how or why it's a sin.

So I ask what is wrong with expressing love for more than one person openly? What's wrong with raising kids in a home with more than two loving parents?

What specifically is so wrong with raising a polyamorous family? Please give me arguments that can't apply to monogamous parents.