1 - 4 should obviously be legal

5 "one thing is not like the others and guess which one it is" .

Seems like people use the word pedophilia to encompass everything between the correct definition (sexual attraction to pre pubescent children) to attraction to people younger than 18, child abuse, sex with a legal minor, etc.

Pedophilia is not illegal anywhere, what is illegal is the sexual act with a person younger than x (insert corresponding age in wherever you live).

Seems like the op forgot to include #6 zoophilia for the shock factor too.

No i dont think zoophilia should be illegal either.

You can look it from two different ways - what is good for personal freedom and what is good for tribe (country) in battle against other tribes (countries), because, historically, that's what it comes down to.

If you look it from freedom perspective, all 5 points are, to various degrees, valid.
If you look it from tribe/society/country perspective, more or less, none of them really are, including same-sex marriage (since main purpose of monogamous version of marriage has always been producing more 'human meat' for rulers, while keeping beta males in check).

So at the end, currently, it all boils down to various levels of random 'morality' and 'equality' our individual countries are imposing on us as their subjects.

I'm so tired of the slippery slope argument. Any two consenting adults can now get married, which is a right that shouldn't have ever been taken away in the first place. Also, this act wasn't taboo until a pope decided to make it so. Also, among those 5 things listed, why are the first 3 illegal in the first place? Polygamy, if multiple adults want to be married to each other, why not? Why isn't marijuana legal? People want to use it, and it hurts you no more than drugs that are already legal. Prostitution, two consenting adults having sex, one just happens to be getting paid. There's no reason prostitution should be illegal. I'm on the fence about incest, it's two consenting adults, but incest can lead to horrifying birth defects. Do you know what sets the other four acts completely apart from the fifth on listed? CONSENT. A child is not old or wise enough to give informed consent, and therefore, sex with a child is equivalent to rape. Pedophilia almost always leads to some form of mental instability for the child, so yes, this act will always be considered taboo.

Marriage to animals, 5 wives/husbands, allowing 10 year olds to have sex with 50 year olds, etc. The limits to all of this will probably be gone eventually.

We have reached the most evil point in history. I only see morals continue to decay. The Bible says that in the end times, people will call good, evil and evil, good. Those times are here.

When people say incest, I always assume it means sibling or first cousin. 

But your response to three is what I completely disagree with. Look at all of the problems with prostitution. The prostitutes themselves are often times seen as less than people, and the "you killed a hooker" joke is so common, that it's not even thought of as offensive. Then there's the std problem, because it's not a regulated industry, and the fact that some people are sold into the sex trade illegally and against their will. If this was turned into a legitimate and regulated industry, like how it is in Nevada, these problems would largely go away. 

Humans are intelligent creatures. They will always use a system to sociologically keep things in check. We dont need religion to have morals. Also, we learn from fallen society about how to treat our fellowman and deal with economic situations. If people paid attention they would understand. We're constantly learning and growing by trial and error.

Women being manipulated into the sex trade happens even in places where it is legal.  Beyond that, I don't really know enough about legalized prostitution to comment.  I did not say that prostitution should be illegal, just that there are potential negatives that are well worth considering.

Is this a serious thread?

If Marijuana became legal in the US it would become useless to the cartels and that would only piss them off. Its like finding a way to stop oil from being the leading fuel source in the world. You will only piss off the establishment. Colorado has made billions in tax dollars from legalizing marijuana. Thats just one state.

Honestly I always image that 70 years ago, if the intenet existed  and the question "Should Blacks be given the same rights?" was asked, the answer you have just given would be the answer someone beck then would have given.