What's the in practice difference between say, a polyamorous group raising children together in a stable situation and a large, extended family with various cousins and so on?

The fact that their internal dynamics are completely different.

Or to make it even simpler, I see no strong reason to say "you shouldn't be gay" when you could be saying "Hey gay guys, you should form a monogamous pairbond and raise children together for 18 years".

Because:

1) The child is deprived of a mother (or father). And yes the two play different roles in bringing up children.

2) Gays aren't monogamous. One obvious way to see this is to note how much gay culture is based around gay bathhouses. Another way is to image search pictures of gay pride parades.