We talk a great deal about about rights, and far too little about the responsibilities and duties concomitant with our rights, and hardly at all about the fundamental basis of both right and responsibility: relationship. Because of this latter deficiency, we have become very confused about rights and duties and think that they require our consent to be binding.
It is of primary importance that we acknowledge that every right that we possess implies certain duties. If I have a right to live, then you have a duty to not murder me. If I have a right to free speech, then you have a duty to let me speak. Similarly, to say that I don't have a duty to, for example, shelter and feed you, implies that you have no right to my house and food.
A right can also imply a duty on the part of the right-bearer as well: my right to free speech also comes with the duty to not speak falsely against my neighbor. My right to life also comes with the duty to not take my own life. My right to raise my children implies the duty to protect, care for, and love them.
Rights and duties, though, are not free-floating entities, but they are consequences of the fact that there exist certain relationships between individuals. It is the nature of those relationships that determine what rights and duties the persons involved have.
Further, that relationship is a relationship among persons. I cannot have a personal relationship with a rock, because a rock is not a person. So there's no such thing as geological rights. Furthermore, it is a reasonable extension to suppose that institutions or groups, like countries, because they are collections of persons, can corporately enter into relationships with individual persons. The nature of the relationship is still personal, though not between individuals.
The classic example of a basic right, is the right to life. All humans possess the right to life because they are rational beings. Because they possess this right, other human beings have a responsibility not to directly kill them. The basis of this right and its corresponding duty lies in the relationship that subsists between all human beings: humanity comprises a community; we are all each other's brothers, each others neighbors, and so, at a very basic level, we are all each others keepers.
Some relationships are not voluntary.
You cannot choose to be part of the human community. You are part of the community whether you want to be or not, by virtue of the fact that you are a human. Similarly, you cannot choose your parents, or your siblings. Yet there exists there a familial relationship, and with that come certain rights (the right to be reared by your parents, and loved by them, the right to be taken care of by your older sibling) and duties (the duty to mind your parents, the duty to take care of your younger siblings). You cannot choose the country you are born into, and yet you have certain duties as a citizen (to follow the country's just laws); and you have rights with respect to your country (the right to justice, the right to share in the common good, the right to be unmolested in pursuit of your private good).
Other relationships, however, are voluntary. When you marry, for example, or when you become a statesman, or when you enter into a contract, or become an employee. All of these are relationships that carry with them certain rights and responsibilities. Because you have voluntarily entered into those relationships, you have voluntarily taken on those rights and responsibilities.
It is in this sense only that duties are optional: if and only if the underlying relationship is voluntary. Once the relationship exists, duties flow automatically from the relationship. You cannot line item veto the duties you have to your spouse, for example. If you marry, you take on all that entering into that relationship entails.
This context provides an avenue for addressing one of the great confusions of our time: abortion. Does the woman have the right to end her child's life, or to have a doctor end it for her?
For this post I will prescind from discussing the unborn child's personhood; for the sake of argument, let's agree that the child is a person.
In all other contexts, aside from pregnancy, we will agree that parents have the right to rear their children, and concomitantly parents have a duty to protect, feed, and love their children. No moral person would agree that among the rights of the parent is the right to end his child's life, or even the right to abrogate his parental responsibilities through abandonment. All of this, again, is because of the special relationship between parents and their children.
But this is not enough to see the wrong of abortion, because one might object and say "Woman's body, Woman's choice", or again, "no one has the right to use another's body, so neither does the unborn." It is true that my right to life does not extend to violating another's rights (in this case, the right to bodily integrity). It has been correctly pointed out that even if your child is dying, he does not have the right to one of your kidneys, and no law should mandate that you give up your kidney for his sake.
If you extend this argument to pregnancy, though, you end up regarding the fetus as a kind of intruder, an external interloper who unjustly "uses" the mother's body and resources to its own benefit. Indeed, some will even go so far as to call the baby a "parasite", thereby unveiling the true monstrosity of their view: not only is it callous and crass; not only does it reduce pregnancy to a kind of economic injustice; it is utterly inhuman and nightmarish, envisioning pregnancy as alien impregnation.
Where, exactly, it goes so wrong is in ignoring a central fact: the child in the womb stands in an absolutely unique relationship to his mother. This relationship goes beyond the normal parental relationship. It is symbolically manifest in the very flesh of the mother and child: via the umbilical cord and placenta, the child clings to his mother, their bodies so intimately united that the mother's body gives nutrition and life to the child.
It is a biological, and natural, fact that the mother's womb exists for the child's sake. To provide a safe environment and proper nutrition to the child is the one function of this organ. Indeed, the mother's body is receptive to, and welcomes the child. This indicates the nature of the relationship: the child, indeed, has a right to be there; the mother has a duty to let him be there. If the child did not belong, then there would not be a place for him in the mother's womb. The child is emphatically not an intruder or parasite.
"But," it is said, "consent to sex is not consent to pregnancy, or motherhood." That may well be true, but nevertheless, when a new life that is blood of her blood, and flesh of her flesh comes into existence, there is now between the mother and child a new relationship, and with it comes all the rights and duties of both mother and unborn child.
Let us not forget the fathers, either. Once the child comes into existence, the man is as much a parent as the mother, though he does not share in that special relationship to the child that the pregnant mother does. Nevertheless, he now has the duty, whether he consented or not, to care for the woman and for his unborn child, and to raise it when it is born.
The woman is now an expecting mother. The man is now a father. The child is their child, and has a right to be treated as such. These are the facts, and consent is irrelevant.