In a perfect world, if two people consent to having sex, they should be prepared to accept the lack of guarantee inherent in all contraceptive methods and the consequences (*cough* kid *cough) that may follow.
Unfortunately, this is a far from perfect world.
Regarding the average therapeutic abortion: if two consenting people have sex, it seems a bit ridiculous for the woman to later be all like, "Oh noes! I'm pregnant! I don't know how that happened...." If you aren't at a place in life where you could potentially support a child, or at least carry it to term and give him/her up for adoption, you probably shouldn't be fooling around. Sex is fun, yes, but is it a right? It feels good, to be sure...but it can have consequences, as do many life choices. It seems to me that the whole attitude of, "gosh, I really want to have sex, but if I get pregnant I'll just abort the kid" is a bit irresponsible. No one, in this scenario, forced you to have sex. You chose to do so, knowing that anything short of a hysterectomy is no guarantee. In my opinion, you should then be prepared to accept the results of your choice.
It is easy to simply dehumanize the mass of cells that forms after the union of a ovum and sperm by calling it an embryo. But we must remember that this mass of cells is not just any old embryo; it is not a gorilla embryo or a (insert random animal here) embryo, it is a human embryo. Given the chance, it will develop into a human being. We must be careful when we say that the embryo is not human, or alive, or what have you, based on the fact that it cannot care for itself, think for itself, reciprocate, etc. Is not the fact that it is human enough? If the definition of humanity is reduced to performance-based indicators, we have fallen down a slippery slope, particularly when it comes to end of life care or those who are terminally ill/disabled. Someone in a coma may not be able to feel love or hate, and an elderly person may not be able to provide for themselves--are they then less human, and therefore disposable if they become a nuisance?
As it has been alluded to previously, pregnancies resulting from rape, etc., are a fairly rare occurrence. They do happen, though, and must be considered. In this case, however, I do not think it is the role of the ED physician to dictate whether the abortion should occur. Though I would personally recommend, depending on the situation, carrying the child to term and then giving him/her up for adoption if at all possible, that is not my choice to make. As with the administration of drugs like tPA, etc., I think that I would feel comfortable making the drug available and discussing with the patient risks, benefits, and ethical elements of the scenario, and letting them come to their own conclusion (or, as has been mentioned, providing a timely referral to a doctor who could accommodate their desires). It is not an acute situation, and the patient can therefore be the primary decision-maker in this scenario.