The question of the morality of creating and/or killing undeveloped embryos (or even fetuses) ultimately is a value judgement, and like religion or drugs, is something we are unlikely to come to a concensus on.
However, I believe that it is not the purpose of the government (and thus the law) to promote or protect morals. Hence I am in favour of deregulation of intoxicants, for example. The question of the legal status of actions such as abortion or stem cells is then phrased differently: Rather than being a question of morals, it becomes a question of the societal effects of a ban, whether or not this is the valid realm of government, and ultimately property rights.
Perhaps humans are inherently endowed with the right to control their lives. But such is not and should not be the basis for the behaviour of our governments; rather, government should endeavour to protect property rights because of the positive effects on society that such protection engenders. Murder, then, is not illegal because it is morally wrong (though perhaps it is --- that is not an aspect of this discussion). Instead it is illegal because in a society where murder was OK, people would live in fear and the general quality of life would be substantial poorer.
What, then, are the societal effects of the actions under consideration? So far as I can tell, there are none. An embryo is certainly not self-aware enough to be able to exercise property rights, and there is no real societal interest in having more embryos around. Thus I argue that even if termination of an embryo is immoral, it ought not to be illegal.
There are three counterarguments to this proposition that I can think of: That this makes a difficult-to-draw line; that this promotes eugenics; and that this would make the murder of a newborn baby acceptable.
Taking these ideas in turn, I begin with the first: If murder is acceptable for something unable to exercise control over itself, at what point does it become unacceptable to kill a member of homo sapiens? At birth? Sometime before? After? Even a newborn fetus is rather unable to communicate self-awareness, though perhaps it has some.
This is, unfortunately, a common question in the law. Consider the following questions: At what point does one become mature enough to consent to sex? At what point ought one to be allowed to vote? How much resistance ought one to make in order to label a sexual encounter "rape"? Does reverse-engineering an iPod constitute a violation of the DMCA? These questions are rhetorical in nature, and while one might disagree with the solutions implemented presently in law, I hope that in general the principle that there exist difficult decisions such as these is acceptable.
Thus, the exact time at which homocide becomes unacceptable will almost certainly depend on exact circumstances, but perhaps that ought not to stop us from defining some particular moment as a defining step. Birth might be acceptable, because although I would argue the societal value of a ban on homocide does not develop until at least some months after birth, such a point is difficult to measure and will certainly vary.
The next possible counterargument is that this might promote eugenics. For exmalpe, it might make it acceptable to kill someone who is mentally insane. While I might agree that from a societal standpoint there might be little impact in killing an insane person, from a practical standpoint doing so is problematic. In a parallel fashion to the question of the death penalty, it is difficult to determine insanity in a perfect manner, the decision will almost certainly involve political or prejudicial elements, and ultimately is irreversible. The extent of discretion that would be required is too easily abused, so it would be better to constrain the state in this way. Note that this argument too is practical rather than moral.
The final counterargument discussed here is that the legal setup suggested would make it permissable for someone to kill another's child, born or unborn. That would be true, except that for practical reasons we ought to consider the unborn (and perhaps born, depending on the answer to the first counterargument) child to be the property of one or more of its parents; thus anyone who intervenes would be depriving the parents of their property rights. I hope that it is not necessary to defend this right on the part of the parents.
This is an idea that I just came up with recently, so if any of you have suggestions, comments, or rebuttals, I'd be glad to hear them. Thanks for listening.
“The problem with that definition is just that it's bullshit.” -- localroger