Well, I'll step up to the plate on this one. I've been involved in these types of relationships before. The first time it didn't work out very well (but it wouldn't have worked out even if there hadn't been other people involved). I'm... not entirely sure what you'd consider the relationships I'm in now to be. It's a bit odd, but it works for me. I'll summarize by saying I can romantically be with more than one person but I limit it to very specific people who are in a very specific, shared situation with me. I can also be sexually involved with these people without being in a romantic relationship. The only limits we have are self-imposed, of which there are a lot; and that we don't pursue people who are outside of our little group. But it works.

I suppose I'll address your questions now:

How do you know you're polyamorous: I'm not sure about this. I don't even know if I technically am. I think it would be very easy to become confused or mistake not being ready for commitment as polyamory.

Are humans naturally polyamorous: I'm going to take a page from my mother here and say that most humans are serially monogamous. However, there are exceptions.

Should polyamory be acceptable and/or legal: Acceptable - yes. I'm not sure if it should be legally recognized though. This is largely due to the controversial cults you've spoken of (Look up fundamentalist mormons if you wish to read more about this. Under the Banner of Heaven was a pretty popular book on the topic.) However, these cults tend to be polygamous which tends to refer to polygyny (one man with multiple wives) rather than polyamorous. In these situations women are used and lack many rights. It's about control. So I'm pretty against polygamy. I've also seen polyamorous relationships as being very fluid. I don't know of any yet (doesn't mean they don't exist though) which are permanent. I'd need to see long term groups actually vying for marriage equality before I considered it a major issue. Most people I know who are polyamorous don't like the concept of actually being tied to people in an exclusive bond. They're often very free spirits who balk at the idea of marriage in general.

What you think poly is: I think it's a choice people make in how to conduct their sex lives. I think many people are naturally drawn that way and should not be shunned for it. I do not think it is a form of sexual orientation since I see sexual orientation as referring to which gender or sex one prefers in a partner. However, I think it can be a very important part of one's sexual identity. There is a hell of a lot more to a person's sexuality than orientation.

I want to add in one more section. What polyamory isn't: This term does not refer to a monogamous person who cheats. I personally don't think it refers to polygamists, which I described earlier. One does not have to be bisexual to be polyamorous and it does not mean someone is hedonistic... I am, but that's beside the point. Most polyamorous people I know are very free sexually, and do not approve of concepts of 'ownership' over one's partner. I'd be interested to see how many self-described polyamorous folks that doesn't apply to though. Like you, I haven't met as many as I'd like.