It is? Where does the constitution address marriage?
It doesn't address that directly however it would be considered under the rights of the people.
The first ten amendments to the Constitution—the Bill of Rights—came into effect on December 15, 1791, limiting the powers of the federal government of the United States and protecting the rights of all citizens, residents and visitors in American territory.
"The Bill of Rights protects freedom of speech, freedom of religion, the right to keep and bear arms, the freedom of assembly and the freedom to petition. It also prohibits unreasonable search and seizure, cruel and unusual punishment and compelled self-incrimination. Among the legal protections it affords, the Bill of Rights prohibits Congress from making any law respecting establishment of religion and prohibits the federal government from depriving any person of life, liberty or property without due process of law. In federal criminal cases it requires indictment by a grand jury for any capital offense, or infamous crime, guarantees a speedy public trial with an impartial jury in the district in which the crime occurred, and prohibits double jeopardy."then why should incest between consenting adults be illegal?
I believe insest is illegal not so much in terms of morals but due to the fact that it is damaging to the human genome. Many royal peoples of other nations did this in the belief it would keep their royal blood strong. When in reality it caused all sorts of health problems due to the lack unique DNA with in the genome. It is the difference that makes us strong. Without a fresh infusion of DNA we would eventually die off. This almost happened to the Egyptian royal dynasty.
Why should polygamy be illegal?
If everyone in practicing polygamy was in full agreement then I don't see much of an issue with it. The problem is that it's not like that and this is due to the fact that the only ones who're doing this are those segregated religious sects. The Church of Jesus Christ of latter-day saints, formally run by Warren Jeffs, is a good example of this. They forced women of young age (teens to be exact) and have then marry seniors who're in their 60's and over. The women do not get a choice in this and it is very disturbing as it violates their rights under the charter. There is supposed to be a segregation of church and state yet you have these nutjobs start up their own religions so they can side-step the law to some extent and do whatever they want to their followers. Religious freedom is one thing. Oppression used under the belief of religion is another thing entirely.