Consider the following logical argument:
1. Actions have naturally occurring consequences.
One of the well-known consequences of sex (even with contraception) is pregnancy. This does NOT mean that procreation is the only purpose for sex. However, pregnancy does naturally result from sex. That fact is inescapable and everyone knows it. Even if some sort of contraception is used, pregnancy may naturally result from sex. So one must be prepared for that possibility.
2. We are responsible for the natural consequences of our actions.
In the real world, we are and ought to be held responsible for the natural consequences of our actions. Even if we do not intend for a particular consequence (such as pregnancy) to happen, and intentionally take steps (such as contraception) to avoid that consequence, we are still responsible for that consequence if our preventative measures fail. It is not unreasonable to require people to take responsibility for the natural consequences of their actions. If those consequences can be avoided or ignored without violating someone else’s rights, that’s OK. However, if that is not the case, then one must be required to take responsibility for his actions.
3. We do not have the right to kill another innocent human being in order to avoid taking responsibility for the natural consequences of our actions.
As premise 2 states, we are responsible for the natural consequences of our actions. In addition to that, we certainly do not have the right to avoid taking that responsibility by killing another innocent human being.
4. An unborn baby is an innocent human being.
That this unborn child is innocent of any wrongdoing should be obvious. Clearly, he is not guilty of any crime whatsoever. In particular, he is not even responsible for being present in his mother’s uterus using the bodily resources that her body (intentionally) delivers to him via the placenta. He had no say in his formation there. So how could he be responsible for that? Instead, he was created there by the consensual actions of his biological mother and father. The mother and father are the ones responsible for the unborn child’s existence and for his need of his mother’s resources.
That this unborn child IS a human is unquestionable. Fertilization produces a new human individual. Only those who are very ignorant of some very basic science regarding embryology deny that fact.
"Although life is a continuous process, fertilization (which, incidentally, is not a 'moment') is a critical landmark because, under ordinary circumstances, a new genetically distinct human organism is formed when the chromosomes of the male and female pronuclei blend in the oocyte."
Ronan O'Rahilly and Fabiola Müller, Human Embryology and Teratology, 3rd edition. New York: Wiley-Liss, 2001. p. 8.
5. Therefore, by consenting to sex, women simultaneously consent to any resulting pregnancy, because they do not have the right to end that pregnancy by killing another innocent human being (i.e. their unborn child).
Premise 1 is firmly supported by observation (i.e. it is confirmed by science).
Premises 2 and 3 are both perfectly logical, and it is upon these principles that laws governing human behavior are based.
Premise 4 is firmly supported by observation (i.e. it is confirmed by science).
Thus, the Conclusion (5) follows naturally and is a logically valid conclusion. That is, consent to sex IS consent to pregnancy. This means that an unborn baby does have the right to live within his mother's uterus - even if she changes her mind. Her "ongoing consent" is NOT required, because she gave initial consent and she does not have the right to rescind that consent if doing so involves killing someone (her unborn child).
By the way, you should note that "consent" does not mean that one WANTS to do something (i.e. to be pregnant). The word consent simply means that one "agrees to" or "approves." Furthermore, this consent need not be explicitly stated, and in many cases that consent is only given in the form of tacit consent. A woman gives tacit approval (or consent) to any resulting pregnancy when she engages in the one activity which she knows may naturally result in a pregnancy. She must give this approval (whether intentionally or not) because she does not have the right to kill someone (her human offspring) in order to end the pregnancy.
Since pregnancy is a natural consequence of sex, claiming that consent to sex is not consent to pregnancy makes as little sense as claiming that consent to jump off a roof does not mean consent to hitting the ground. One thing naturally results from the other - even if precautionary measures are taken.
Also, folks have suggested to me that this must mean that if one gets in a car, then one consents to be in a car wreck because a car wreck may be a natural consequence. This is absolutely correct! One does, in fact, consent to the possibility of an accidental car wreck every time one rides in a car. This does not mean that a car wreck need be inevitable, however. As with pregnancy, one can take precautionary measures (driver’s education classes, etc.). Nor does it mean that a driver is justified in intentionally causing a car wreck. However, an accidental car wreck is a natural consequence (though rare) of riding in a car, and one must be prepared for that possibility. Otherwise, one should avoid riding in a car. Likewise, if one is not prepared for pregnancy, then one might want to consider not engaging in the one activity which naturally results in pregnancy.