Contracts, Abortion, and You

One of the most common arguments, against both libertarian outcry that "taxation is theft" and against abortion, is the 'tacit consent' argument. Essentially, it is posited that by doing some action (in the case of the State and taxation, it is remaining in the country and using tax-funded goods and services; in the case of abortion, it is having consensual sex), you have actually tacitly contracted to do something further (pay taxes or not have an abortion).

Now, I think this is a bad argument for several reasons (and in the not too distant future I hope to elaborate more on this), not the least of which is that these seem like highly improable cases to even consider for a tacit consent argument. But I think one of the biggest problems with such arguments is that they ignore the inalienabilty of our rights. To say that our right to self-ownership is inalienable is to say that that nothing we say or do (or contract) could possibly give up our right of self-ownership. (This is why, for instance, I would argue that on libertarian principles one could never sell oneself into slavery.)
The importance of this point to the matter at hand is that contracts are always 'breakable', in the sense that even if I contract to do X, I could always change my mind. I may owe compensation to the person with whom I contracted, but that person would have no right to force me to do X. Thus, e.g., I might agree to mow my neighbor's lawn. Then, the day that I'm going to mow it, something comes up and I can't do it. I may owe my neighbor compensation for not mowing her lawn and breaking our contract, but my neighbor couldn't use force to make me mow my lawn.
I think this point is apllicable both to arguments against social contract theory, as well as arguments in favor of legalizing abortion. Focusing on the latter, let's assume that a woman having consensual sex is tantamount to 'tacit consent' to bear any child that may come of her actions (I think this is a very weak argument and hard to maintain, but that's a different matter). I still don't think this would be a strong argument against abortion, since the woman has every right to change her mind and break her "contract." Since no contract can take away a person's inalienable right of self-ownership, her 'consent' to bear the child doesn't bar her from the future action of having an abortion.