I understand your frustration. I live in Iowa, so it's completely legal for me to marry a woman, if she's single...But I'm really cautious about who knows that I'm dating a married man. (And I've had some really odd conversations, like the one where my manager asked how long I'd been dating the new guy she saw me with, and I said "Oh, him- he's not new, and we're not dating, I just live with him...and his wife and kids.")
So, there's times when it does hurt to feel like a dirty little secret, but the alternative is to either to deal with negative reactions, or not have those people in your life. If the people you're keeping it secret from aren't part of your everyday life, it doesn't really matter...and it's not necessarily forever either. Social change doesn't happen as fast as we want it to, but it does happen, and poly had been getting media attention lately, which raises awareness- and if someone is aware of poly, that takes the someone else's problem field off of poly relationships around them.
Maybe down the road a little, you'll gradually get to be more obviously part of her everyday life, and people who know that poly exists will start noticing the dynamic between you, that you're together a lot, that you make lots of comments on each other's facebook- and one day, someone who you didn't come out to because you assumed they would react badly will say "I don't mean to pry, but are you in some kind of poly relationship with X, or are you just really good friends and I'm misreading things?"