>There are plenty of secular arguments for flat earth. It's not about the number of arguments, but about their validity. There are no secular arguments that don't depend on logical fallacies or invalid assumptions.
This is a false equivalency. Flat earth is a matter of facts, not ethics. Further, you are taking the least charitable interpretation of my words. Of course I meant that there are plenty of _valid_ secular arguments.
>And similarly when talking about forcing someone to donate their blood, or part of liver for transplant we'd need to balance the rights of the forced donor and the rights of the prospective transplant receiver, right?
Yes, this is a common counter-argument. I first encountered this in A Defense of Abortion in the form of the "unconscious violinist". A more whimsical version can be found in Rick and Morty's second Interdimensional Cable. A common counter-counter-argument is that this situation differs from abortion in that the mother bears partial responsibility for the creation of the fetus, and thus owes it more than she would owe another random person. This is one of the reasons people get more outraged about restriction of abortion rights in cases of rape, as the mother had no volition and thus (by this logic) owes her fetus nothing more than she would owe a stranger.
Regardless, you may find that the right of the mother to bodily autonomy outweighs the right of the fetus to life, but not everyone will weigh the situation the same. We often agree as a society that one must help others if the cost to oneself is considerably lower than the value provided to others. For example, you are obligated (in an ethical sense) to provide water to a man dying of thirst as long as you have plenty. Or perhaps (more controversially), you are obligated to jump into a cold swimming pool to save a drowning child (if you can swim). Some argue that the temporary albeit serious inconvenience to the mother outweighs the life-long value to the unborn child in a similar manner.
>It's not only not one of the challenging ones, it's not a serious question at all. Modern ethics is pretty clear on reproductive rights.
The funny thing is that each side seems to think this, and that the other side is insane, naive, or misguided for disagreeing. FWIW, I am personally pro-choice, but I also think that if you cannot understand the other side's arguments, that is a weakness of your own mental model, not a sign that they are crazy or bad arguments.
I'm happy to talk more about this, as I think it is a genuinely interesting and challenging issue that remains unresolved, unlike many things that our society argues about. Another interesting unresolved issue is the ethics of punishment - I have come to believe that government punishment is generally unethical, but that's another post.