I definitely didn't think it should have been closed, but I think the reasoning was that it was pretty unrelated to polyamory. I guess an analogy would be someone asking about dogs on a cat forum. Sure, they're both pets, but there are forums for dogs, and this forum is obviously about cats!
But since there are many people who own and/or love both cats and dogs, I would say that if the thread is actually being posted in and all, there is no reason to lock it.
And if it's not being posted in, there is no reason to lock it either since it will just fall off the front page and not be relevant.
But we do get a lot of people who think polyamory is just about sleeping around, so I'm guessing that's why it triggered the moderation wrath?