The difference between same-sex marriage and polygamy and incest are this...
I am a gay man. Currently, in the eyes of the government, my sexuality is not seen as equal to that of the hetereosexual man next to me. By not allowing me to marry who I want, or by referring to my relationships as anything less indicates that I am not equal to the rest of you. It is basically saying that me as a person, as an individual am not validated in the eyes of the law. I was born this way, it was not a choice, thus I cannot simply "choose" to marry a woman. Separate but equal is not equal. If were to get a civil union, I would not call it so, I would call it a marriage, and I would say I have a husband. Referring to same-sex marriage as anything other then marriage is sickening. It's like referring to a marriage between a black and white a "mixed race marriage."
Polygamy and incest are choice relationships. The individuals involved are CHOOSING to enter into these relationships. A brother is not born to instinctually seek out his sister for love. A man is not hardwired to love three women at the same time. Gays and lesbians are BORN to love members of the same sex.
I am not for the polygamy or incest because they have a choice, I don't have a choice. What I want is my relationship, my life, my livelihood, my individualism to be not only recognised but protected under the law so that I can live a full and rich life as I was entitled to by birth.
If any of you opposers can gather at least a semblence from my post, the message is about validation and recognition of the way I live my life. It's about me wishing to be treated as an equal, as a peer. Same-sex marriage is about recognising that gays and lesbians have no choice just like straight people have no choice, in who they love. We did not decide one day that we wanted to love members of the same sex, just as straight people did not decide one day to love a member of the opposite sex. Sexuality is not a choice, just as skin colour, hair colour, eye colour, etc is not a choice.
Until you have realised what it is like to live your life seen as a second class citizen, you will never understand.