Genuine Question?

Why is having a poly relationship in marriage seen as a taboo thing? I still do not understand why it is illegal in most-if not all-states. My info might be wrong and I am a bit ignorant maybe? I don't know many things as I live under a rock (this is a saying)-

I'm dating my GF and my BF. I want to marry them. If I can not marry them, I want to get them promise rings and stay dating them until I am gone. One is long distance, one is closer to where I live and I see them often. I love them. It sucks to see that people see it as cheating or disgusting-something wrong- I assure you I love them both. They love me. It is consensual.  

So why are these kinds of relationships seen as wrong? Please answer to help my confusion ^^;;