The famous violinist thought experiment:
You wake up in the morning and find yourself back to back in bed with an unconscious violinist. A famous unconscious violinist. He has been found to have a fatal kidney ailment, and the Society of Music Lovers has canvassed all the available medical records and found that you alone have the right blood type to help. They have therefore kidnapped you, and last night the violinist's circulatory system was plugged into yours, so that your kidneys can be used to extract poisons from his blood as well as your own. [If he is unplugged from you now, he will die; but] in nine months he will have recovered from his ailment, and can safely be unplugged from you.
Judith Jarvis Thomson famously postulated this as an analogue for abortion when the pregnancy is a result of rape. (although the same logic can apply to all abortions) that the use of someone else's body as a means for your own sustenance is a kindness
provided by that person, and not your right
This thought experiment need only be altered slightly to apply to questions about who "gets a say" about maintaining or terminating a pregnancy. Suppose, you and your buddy, after listening to this violinist's music decide that you really want the violinist to live, and your buddy happens to have the right blood type. So you go to the violinist and offer to hook him up, and of course he accepts. By that evening, your buddy is hooked up to the violinist and all is good. Of course, two weeks later... Your buddy is getting sick every morning due the extra work put on his kidneys, and hates dragging this violinist around with him everywhere. He decides he no longer wants this burden. Now, should your buddy have to ask your permission to unhook himself?