Moral Obligation and Surrogate Motherhood

Another question from C.Ewing:

This brought up the periodic question you seem to have of just where the moral imperative line is drawn. It seems to me that "super" just can't be applied to ethical mandates. At any given time you either are or are not meeting your ethical obligations, and often enough times we all fall short, but we weigh what seems "most" important, and those which we deem too bothersome, we let slide. Of course, in this process it seems obvious that we're opting not to do things that ultimately we really know we should be doing.

This, however, seems an exceptional case. When your friend wants a bit or your ice cream, even if you really loves you some ice cream, you let him or her have a taste. You let your little brother (thanks, Mike) borrow your SNES on Saturday morning, even when you would rather sleep, because you know he just has to beat that next level. We will readily be "put out" so to speak, when we are able to do so, because we know we can do so, and should. But this is all stuff. These are mere inconveniences at best.

But can you be obligated to "loan" your body? This seems downright bizarre. On the one hand, when a couple really wants a child, our hearts ache for them. I'm not even a parent, and I sympathize with the plight of a childless would-be mother. But it seems--at least, I think--a different sort of case here. Surely, we should always do what we can. But giving someone a lift home from the bar, and loaning out your womb seem to be wholly different things. Are they? Do we draw the line when it comes to borrowing bodies? The only things I can think of which are similar to this are organ donation and giving blood. But giving blood is, again, seemingly just an inconvenience, save for those few of us who have genuine health related issues with doing so. Organ donation occurs post-mortem, and so that hardly seems an issue unless religious questions come into play (same with blood in some cases). But, foregoing Jehovah's take on this: what about your womb? If it's not otherwise occupied, is there any way in which we can fathom an obligation for surrogation? Sorry, I just had to use that one.