To the OP:
I do not believe there is such a thing as something “objectively” wrong or right morally. Things are wrong or right because Hashem says so, end of story. Even the examples that you brought of murder or robbery will depend very much on the society you live in, as yacr85 put so well. For example — if you believe robbery is morally wrong, are there ever any exceptions? What about a “Robin Hood”, who steals from unworthy rich people help the worthy poor – is that moral? What if one is stealing to save their life or the lives of loved ones? What if the stealing takes the form of slick negotiating skills in business, and you “earned” that money by being smarter than the next guy? What about those who view our entire tax system as a form of legalized stealing via redistribution of wealth? What if those same people then cheat on their taxes to protect what is “rightfully theirs” from government theft? My point is that it is very easy for people to come up with all sorts of justifications for even the most obviously “immoral” activities, and who are you to say that your perspective is more right? The same thing can be done with even something as extreme as murder. Many societies throughout history felt it was morally RIGHT to murder babies who were sickly or handicapped so as to avoid burdening society with their care (and even today many feel that way with regards to aborting a baby with developmental delays). What about in times of war when you are fighting to defend your country? What if you believe the defense of your country requires complete destruction of your enemy? Was using the a-bomb to end WWII morally right or morally wrong? What about the death penalty for criminals? What about Halachically sanctioned or required murder (goel hadam, ir hanidachus, milchemas amalek)?
Mishakav zachar is immoral for the goyim because Hashem said so, and the only way to have “objective” morality to is to look at how Hashem defines it. (I do believe the case against toevah marriage can be made from a logical perspective as well, but that is not the bottom line of what makes it right or wrong, as most “moral” questions can be argued logically in both directions!)