Currently Online 
I feel like all of your points outside of point 5. have some merit of legalisation to them. Obviously everything that is legalized needs laws and regulations to prohibit abuse and missuse of the system, but establishing concise rules about these topics can be very beneficial imo.
1. I already said in the Polygamy thread that, as long everybody involved is a consenting adult and has equal rights regardless of gender, race, age, sexual orientation and other factors, it should be completely fine and a recognized form of human unions. Obviously in a polygamist family struckture that involves children living within it, firm and concise child protection laws need to be put in place, to prevent even more messy breakups (because more people are involved) as well as any form of abuse from adults of the group in general. (In some polygamist patriarchal cultures it wasn't uncommon that the main wive would physically and psycologically abuse children of the other wives out of spite/jealousy for example, without the other wives being able to protect their children from her. Arguably thats a bad example because most of the wives in these societys werent exacly consenting to the polygamy anyways, but love is complicated and can turn ugly, so protection and regulation should be in place.)
2. Again, as long as regulating laws according to the farming, quality standarts and controlled distribution only to adults are given, I don't see the problem with it being legal. The Netherlands have not fallen victim to drug corruption of the souls in the ca. 40 years marijuana has been (semi) legal over there and neither will the rest of the world. Distribution to you teenagers should be prohibited though, since recent studys have shown it to cause irreversible alterrations of the brain, if consumed at such a young age.
3. is legal in my country. Has been for ages. And here comes the tricky part, it doesn't really work. Like with all other points I'd say if the right laws where in place to protect the adult consenting parties from any kind of abuse it would be fine. And in my country a lot of the right laws are in place, but some are missing still, yet theres massive problems with pimps (a concept that should be ruled out by good legal regulation), forced prostitution, underage prostitution, human trafficking, underpayment and horrendus working conditions. This is because the laws that are in place do not get enforced properly and there's not enough laws in place to protect the prostitutes and their customers, not enough laws in place to cut out the middlemen. Prostitution is still stuck in this dirty little corner nobody likes to talk about, it is still highly exploitative work for a lot of men and women, with no proper health standarts in place to protect either the prostitutes or their custumers, but as long as they pay their taxes the states is fine with it. This is NOT how you do legal prostitution.
My impulse is to say there should be an official registration process for every brothel as well as the prostitutes themselves and official licenses and healthcare checks involved. Possibly Unions, work contracts and pricing standarts as well. But most of all it needs to be visible, in a positive light. My fear is that this would only create a clean branch of the industry while all the human trafficking shit forcing young girls and boys into prostitution will just go on it's merry way. That drug addicts would still turn to illegal prostitution to support their habit and someone will take advantage of them. But at least prostitutes and their custumers would have the option of a more clean and regulated avenue in a sort of vote with your wallet kind of way? Obvously the really disgusting shit, like child prostitution must stay illegal and will despite that continue to happen as long as there is a market for it.
4. This is where it gets tricky. For a number of reasons. Again, marrying and having kids with your bloodrelated cousin is not only completely legal, but considered normal in my country. It's not stigmatised in the slightest. Any relation closer than that is prohibited on medical grounds and can not be reconized through marriage, because the close relation could cause severe damage to potential children.
You don't even need to have that close of an incest to see the possible effects of this. There are quite a few instances of cousins marrying among each other and procreating a few generations back in my family. For the last four generations, every generation, at least one family member has shown signs of schizophrenia. We believe that those two instnces are related even though we don't have any real proof, however neither my in my gradparents nor in my parents generation people married their cousins and so far no one in my generation has exibited signs of schizophrenia. Could be coincedence of course.
So the first problem is offspring with birth defects of varying severity.
The other is abuse in form of manipulation and forced ralationships. In case you have grown up with your close relatives, they are going to have some sort of emotional leverage over you. There might be gray areas but I doubt there can ever be true consent where parental incest is involved. The parent you grew up with will always hold an emotional position of respect and power in your live and even children of the most shitty parent mostly want to be loved and appreaciated by them no matter how angry they are at them otherwise. This imbalance of power would create a breeding ground for sexual abuse and abuse of power if parental incest was legal. The same applies to older siblings, but in a much lesser degree.
Consentual incest, even if illegal is not prosecuted unless children are born from the union in my country. In which case usually the male is prosecuted and gets jailtime. It's by far nor the ideal solution and while I don't think sibling couples should have the right to have biological kids maybe they should be given rights to marriage and adoption. Certainly stigmatisation doesn't help them in any way. Parental incest should stay illegal imo as it opens up too many possibilitys for abuse.
5. Pedophilia, if acted upon with a real child or in a way that indirectly meant harm to a real child should always be prohibited. Children can not consent. This goes back to point 4. Adults hold a position of power over the child physically and emotionally as well as possess the cognitive ability for manipulation that especially young children just do not posess. Very few children are able to speak up/defend themselves or get out of an abusive situation on their own accord. Hell very few adults can. I'm all for not underestimating children, but it is a simple fact that, lacking the life expierience of an adult they cannot see the consequences of their actions adequately enough to be able to consent.
That being said, pedophilia might be a simply unchangeble sexual orientation on the spectrum of sexual orientations humans are born with, albeit with the unique variable of being one where the other party can not consent and thus should not be engaged with. As with the other points stigmatisation probably doesn't help, instead we should maybe focus on creating a saftey net of couseling, self-help groups and attentive social cicle. Also as with other sexualities pedophilia in most cases isn't the only sexual orientation these people posess, they might also be attracted to adults, teens or whatever else. In these cases they might be able to lead a completely satisfying sexual and emotional life with a consenting adult partner which would be preferrable. These people have to learn how to live with, but not to act on their sexuality, because your personal freedom ends where anothers well being begins.
Besides all this I just wanted to mention that historically the idea that homosexuality is somehow wrong, is a fairly new one, and has been advocated in the western world only for the about the last 15 hundred years or so. Before that homosexuality was accepted to varying degrees in almost all of the ancient advanced cultures. Although in most of those cultures it was only seen as part of a persons sexual spectrum instead of the categorasations we make today. The ancient greeks had, contraty to popular belief only a very limited acceptance of it, that had to take form of a teacher scholar relationship. The romans were ok with it as long as you weren't a lesbian, mostly because women were not expected to have a sexuality at all. Even the early christianity was fine with it .
The old egyptians were also pretty ok with incest too.
sorry for the monster post, btw
