I don’t think it’s a hindsight issue. People discounted the potential of adult stem cell research because they needed to in order to be consistent with their political objectives.
As to the secular argument against embryonic stem cell research: embryos are both human, and alive. Both facts are undisputedly true as a matter of biology. So the general rule against killing humans should apply unless some other exception is implicated. What those exceptions are is a political issue, even if it happens to also be a religious issue.
Note also that the secular justification for abortion doesn’t carry over to stem cell research. With abortion, there is the argument that, whatever the nature of the embryo happens to be, it doesn’t outweigh the mother’s right to bodily autonomy. But these embryos are already removed and frozen. Destroying them for research requires a different justification than destroying them for purposes of abortion
Tumors are genetically human and biologically alive at a cellular level. Does God want us to stop doing cancer surgery? Do tumors have souls? Embryonic stem cells tend to create tumors, not humans, if attached to existing human tissue. Do some tumors have souls while others don't? Where did the soul go for an embryonic stem cell tumor if it doesn't have one?
The pain and suffering argument (not that you've raised it) is nonsense as well, since pain and suffering during the murder of other animals in socially acceptable contexts is not regarded as very serious, except (sort of) by the Jains. And those animals, even insects, have more cognitive/sensory capacity than a human embryo (which has zero).
The only foothold the religious argument can rely on, without falling to oblivion, is the notion of future potential of a human embryo. But that raises other issues. The embryo won't get there on its own. It requires 9 months of symbiosis with another human with her own interests, or perhaps in the future there will be some option of 9 months of extremely expensive artificial incubation.
When you descend into the insanity of judging based on (uncertain) future potential, it's unclear why, as a matter of religious doctrine, destroying/wasting embryos/zygotes is forbidden, but wasting gametes is perfectly acceptable. If a woman's interests, ability to support a child, and relationship with (potentially) a spouse (who will co-raise and co-support the child) are not relevant in the determination of whether a future child is worth seeing through, why then would you stop at banning abortion? Why stop at banning contraception, even? Women, if maximizing potential future children is paramount, should never ever be allowed to say no, to anyone, ever, should they?
Of course, the reason for that difference in treatment is the (convenient) concept that the soul enters upon formation of a zygote, but that is a purely religious concept, and now we're back in religious la-la land, not in secular-argument land like you claimed. So about those tumors with souls...
Regarding the motive for the embryonic stem cell ban — it's not clear that the actual religious objection is primarily concerned with preventing destruction. Research involves splitting and growing the cell lines, which results in branches of the cell line being experimented on and then killed. If each of these isolated cell clusters has a soul, isn't ongoing experimentation just as bad, and actually worse, than simply killing the single master cell line? The latter would result in one death. Ongoing experimentation results in potentially unlimited deaths.
It seems like the real intent behind the federal funding ban for research involving new embryonic stem cell lines is to prevent their creation in the first place, knowing that once created all the options are bad (according to religious dogma) and result in the destruction of one or many soul-vessels. But to be ideological consistent, wouldn't you want to prevent the creation of never-to-be-gestated soul-vessels by splitting, too, in addition to preventing their initial creation? If so, W's ban makes no sense.
If you agree that identical twins have separate souls and tumors don't have souls, I don't know what you (or anyone against embryonic stem cell research or IVF which has similar issues) are doing, other than grasping at biological partial-realities trying to defend your religious dogma.
As to the secular argument against embryonic stem cell research: embryos are both human, and alive. Both facts are undisputedly true as a matter of biology. So the general rule against killing humans should apply unless some other exception is implicated. What those exceptions are is a political issue, even if it happens to also be a religious issue.
Note also that the secular justification for abortion doesn’t carry over to stem cell research. With abortion, there is the argument that, whatever the nature of the embryo happens to be, it doesn’t outweigh the mother’s right to bodily autonomy. But these embryos are already removed and frozen. Destroying them for research requires a different justification than destroying them for purposes of abortion