There’s no “right side of history”. Past attitudes, values and mores will be judged by each culture on the basis of its own moral values. Since those are bound to change over time, so will the judgement made about past issues. You think of course that there has been a “progress” because you hold the current values of your culture as (on the overall) the correct ones, so obviously you will see the path leading there as an improvement. That’s a bit tautological.
I suspect that you believe that values will keep going in a direction that you approve of. Which is IMO wishful thinking. Especially since so much of our values are based on pretty arbitrary moral assumptions that seem “obvious”(or at least very convincing) to everybody only because they have been one could say “brainwashed” into seeing them as positive, and the more “obvious” they seem the less opposing discourse will be tolerated and opposing arguments considered. A lot, probably the majority, of people will refuse to seriously consider the arguments of the opposing side about a divisive issue (say, abortion), and this is even truer for not divisive issues (say, pedophilia) where arguments will be immediately dismissed.
But things change over time, and unless the issue can be settled by objective, scientific facts (say, the earth is round, not flat), and social sciences, psychology, etc…at this point in time definitely don’t qualify as hard sciences whose conclusions can be assumed as factual and not influenced by cultural beliefs (see for instance how the views of psychiatry wrt homosexuality has changed, unsurprisingly following the changes of perception in society), arbitrary moral preferences will decide what is right and what is wrong.
Can you demonstrate objectively without resorting to arbitrary moral statements that the preferences of the mother should trump the right to live of the fetus? Can you demonstrate in the same way that in a society open to it, informed and consensual sexual relations between children and adults would necessarily be harmful? Can you demonstrate that benefiting from the work of others (capitalist system) is moral? Can you demonstrate that animals shouldn’t have the same right to live as humans? Can you demonstrate that the individual freedom of one individual should trump the interests of the many?
You would probably be pretty unhappy in a society that bans abortion, condone pedophilia, forbid salaried work, ban the consumption of meat and limit your individual freedom to benefit the collective. But for someone raised in such a society, all these things would be “obviously” moral. And if you disputed them, you would considered a truly horrible person. How can you say that an innocent fetus should die just because its mother would like it better this way? Why do you hate so much children and pedophiles? Why do you support enslavement of people for profit? How can you even think of killing an innocent animal, let alone of consuming its dead flesh? How egoistical can you be to think that your personal preferences should prevail when they harm millions? And of course, they would come up with a bazillion of “obvious” arguments against your positions that they have learned from their parents, their peers, their schools, their medias, etc…
And it’s not like they would be wrong and misled. It’s simply that they would have different moral premises and it would be in most cases impossible to determine objectively that their set of moral assumptions is better, or worse, than yours. In order to debate with someone of whether something is moral or not, you need a common moral ground. Once the basic moral assumptions are two divergent, no lesser moral issue can be settled. And disagreement on most societal issues is generally based, not on one side being objectively right and the other side being objectively wrong, but on the lack of a common moral ground (even assuming that you know what effect lowering taxes will have, and in fact you don’t, you can’t agree on whether lowering them is a good or bad thing if you disagree on the desired outcome). If people don’t agree about who or what has an imprescriptible right to live, there’s no way to determine whether abortion or meat eating is moral, or regrettable but acceptable, or a monstrous crime.
And I’m generous when I say that what is perceived as right or wrong is decided by a moral reasoning. Pretty often, morals are invoked for arbitrary cultural preferences not based on any kind of moral reasoning. For instance, someone will find normal that a woman will be sent to court for being topless in public, and at the same time will find oppressive that a woman will be sent to court for not covering her legs to the ankle in another country. So, what is “moral” in this case is simply “what is done around here” and what is immoral is any deviation from this norm in either direction.
And of course, finally, even if you were somehow able to demonstrate that something is objectively and indisputably immoral, it’s no guarantee that a future society won’t accept and condone it. And consider it as perfectly moral. So assuming that there will be a “good side of history” and a “bad side of history” is, in my opinion, deluded.
To finish, I just so happen to have read yesterday the blog of a social activist who is also an antispecist, and who definitely see her fight against specism as a natural and obvious continuation of her fight against sexism and racism. For her, who, from her arguments, believe in this “progress” you’re assuming, this progress will inevitably lead to the triumph of antispecism, and people supporting the consumption of animal products, like me and probably you, are definitely on the bad side of history (and if her opinion eventually prevails, we’ll be seen as not just wrong, but horrifically evil, probably as worse than for instance people who kept supporting slavery when its morality began to be seriously disputed).