Forgot to answer the question:
I don't see a problem with any activity that involves consenting adults without doing harm to others.
So, one through three are completely fine, while 4 might also be tolerated, but keep in mind that incest was made a taboo due to a high incidence of birth defects in their progeny. So, in that case, harm could easily befall an innocent person.
5 of course is out of question, as it involves children, and they cannot enter an adult consentual relationship.

My moral standards as it pertains to other people begin and end with "If it's consensual between two like-minded adults and not hurting anyone, then I don't care what they do in their spare time."

1-3 are fine with me.

4 is very iffy, as a birth brought on by incest can bring health complications to a third unwilling participant (the child).

5 is an absolute no, as it's pretty self-explanatory.

" 1. Do you think that thanks to big steps taken to legalize practices which were previously Taboo, that the practices/acts mentioned above are much more likely to be legalized?"

No, that's the Slippery Slope Fallacy. There is no ethical argument that can be made on behalf of sexually exploiting children. Recognizing that homosexuals have the same Constitutional right to marry as heterosexuals has nothing to do with pedophilia. Having that on your list is simply doing a disservice to whatever point you're trying to make.

"2. Do you personally think that morally we as a society as from now onwards if we do legalize such acts, are heading in the wrong direction? Therefore do you want to see the acts mentioned above be legalized?"

If outlawing it causes more societal harm than legalizing it, the law needs to be revised. Some examples on your list objectively harms society. Others on your list do significantly less harm to society than outlawing them.

@Likening Polygamy with Same-sex marriage:

 In the article referring to polygamy I used in the OP, the writer used a nice little phrase which Iiked "Any argument of allowing Same-sex marriage can be used to allow polygamy and it will have the same strong effect" ( Paraprashed since I didn't actually go and look it up again ). If #lovewins and marriage should be between consenting adults then why does Polygamy still remain illegal? People who partake in a lifestyle including polygamy hurt no one so I see no reason why it should be illegal if we have also allowed Same-sex marriage.

The slippery slope fallacy has been shown to be wrong over and over again.  Each case on this list should be judged on its own merits.  We do not, or at least should not, legislate morality.  We legislate based on harm.  

Look over your list, and see which cause harm.  Five can be eliminated immediately.  Children are simply not capable of making such decisions and defending their own self interests.

 Four is pretty clear, but a little less so.  We know it is genetically bad to reproduce, and we know it can easily lead to abuse of power in immediate families.  Of course, what incest is varies from culture to culture.  Pretty much everyone agrees that parent/child incest is clearly harmful, but marrying your second cousin (or even first) varies quite a bit.

Three is kind of iffy.  You can probably make a case for harm based on STDs, and the danger of women being dragged into the industry.  The act in and of itself may not be harmful on an individual basis, but it may be harmful for society at large.  

Two should simply be legal.  It causes far less harm than similar legal substances, and the harm caused by its prohibition is greater than the harm of the actual substance.  

I don't really know enough about polygamy to speak of it or how it effects children.  What I would say is that it makes for a complicated legal arrangement, that would not be very workable.  

What many fail to realize is that nobody gave a shit about homosexuality until religion started preaching their "moral" bullshit. In the meantime a prophet of a certain religion married a 6 year old girl and had sex with her at 9

There are a few issues with polygamy.  It is a legally complicated arrangement, that would lead to a lot of courtroom disputes with things like inheritacne etc.  It's also quite common in subcultures which allow polygamy for women to be coerced into it.  There are also societal benefits to a monogamous system which may explain why it came to be the norm in the western world.  

Humanity would do well to get over its hangups on morality and deal more in reality, despite the shitty rhyme.

1-3 : I have no problem with, if there is consent and understanding from every party !

4 : cousin marrying is autorized in some country, the only bad thing its have baby.... but after all their choice and won't care much. (and from bible we all kid from adam & eve so.... :p)

5 : Will never happenend, kid can't choice !


And for finish, if ppl would stop look at other and make their own buisness, t here would be a lot less war and more peace...

Just because something is legally trying.. doesn't mean you don't do it..     We have many laws that cause issues for our courts..   Our tax code is a prime example, same for medical malpractice,  etc..  Lots of things are difficult to deal with..  we dont make them illegal.