As someone who's in a polyamorous relationship, I'm disagreeing a bit with the statement that poly folks are really good friends who have sex. "Polyamory" literally means "many loves". If someone has more than one friend they're having sex with, that might not be polyamory. It's having more than one friend you have sex with. Some people do use polyamory to describe that as well, but... personally, I understand polyamory to be *being in love* with more than one person. On the other hand, you could say deep friendship is a form of love, so it could be the same thing...
As for the marriage thing, being in love with someone doesn't necessarily mean you're married to them. (And I know some marriages that haven't got a thing to do with love at all.) Being in a polyamorous relationship doesn't necessarily mean you're having sex with all your partners; I know a few poly folk who are asexual. Not having sex with your partners, and/or not being married to them, doesn't mean the relationship is any less important, or any less polyamorous.
Polyamory just refers to loving more than one person, and there are many different ways to form those relationships. In my personal case, my husband and I were monogamous for several years, but it didn't "fit right" for me. I now have my husband and a long-distance committed boyfriend--which restates my point about sex not always being part of polyamory, given that it's kinda hard to have sex from 900 miles away. So we did do the "two people bring in a third" thing (though my boyfriend is mine, not my husband's). But I've known others who started as single and began dating--and fell in love with--two or three other people who have no connection to each other whatsoever. And a few who started as single, began dating and fell in love with two other people, who then met and also fell in love.
I can't remember the rules about linking to other forums here, but I believe it's frowned upon, so I'll play it safe. However, there are at least a couple of forums for poly folk, with a wealth of information about the many ways polyamory can work (or not work, in some cases). If you google "polyamory forum" you'll probably find them. Even if you aren't poly, it might help to read some of the threads there to get a better understanding of it.