What is being argued here is that under moral relativity what is good is determined by the desires of an individual, therefore a rapist would see rape as a good outcome. The idea that dysfunction = a specific thing is also matter of interpretation. The Romans who created the longest lasting empire for example waged warfare continuously. Something that would be seen as obscene today. If your morality is based upon practicality and what works, then a society who's economic basis is built upon slavery (The Romans) would view slavery as good. If that's the case it's impossible to disagree with them because the basis for what you determine to be good, is that which has a good outcome. Which given the aforementioned example, under your worldview it would have calling slavery a good thing. In order to call the Romans wrong for building their empire on the backs of slaves you would need a reason as to why slavery is wrong both for them and us. In which case in order to do so you would assume a moral standard (something not relative).
It's not weirdness, under a naturalistic framework society come about through beings who evolved. If society is comprised of beings who evolved, then the morals of that society came about through evolution as the individuals in your worldview who determine what morals are acceptable in the society have the origin or cause of their morality due to evolution.
Yes it is like asking that. If it's going to be as if you don't exist 150 years from now, then you don't matter now. That which is transient has no inherent value, the only way transient things find value is if they produce or impact something that continues to exist. If the basis for what matters is the gratification of sensory pleasures or inward desires like posted above, then you need to have a reason as to why those things matter rather than just assuming that they do.