As far as your question on if someone else is in your situation I know there was a thread recently. http://www.polyamory.com/forum/showthread.php?t=26159
It is about a man who is his wife's primary relationship in all ways but one. The sexual. If that is what you and your husband want then that's working for you and fine.
As for 'being a piece on the side'. Why would you think it would be like that? I am married, more than 17 years, bf and I have been together now just over a year. It's a long distance relationship but I wouldn't classify it as 'a piece on the side'. For me, at least, I don't quantify the love. We are in contact every day, face time, IMs, texts. He knows of my kids and they know of him, he's talked to them and they've talked to him, but my home life is me and my kids and hubby. We aren't considering bf to move in or become a 'primary' in that way, but he is far from a piece on the side.