Wrong.
What is illegal does not, ipso facto, become "unpreventable."
Your "logic" would also posit thus: "When the speed limit is 55, then drivers will be forced by the Government to drive 55."
More important, however, is the question which you wish to avoid: "Should positive law enshrine certain principles of natural law?"
(We shall assume for the sake of the argument that induced abortion is contrary to the natural law.)
The answer to the question is Yes. Resoundingly, YES.
This also applies to fudgepacking, adultery, and pornography, whether "child" or not.
Whether you like it or not, the Positivists in the Supreme Court have proceeded logically from Roe to Kelo; in Kelo, all they had to do was re-define certain words.
One can characterize Kelo as a manifestation of a lack of integrity, which lack of integrity springs from the willful contravention of natural law.
It's all very well and good that you think "abortion" is a States'-rights issue--but you are wrong on that premise, as well, for the same reasons.
Worse, your 'forces a woman....' argument is really just childish foot-stomping, even if you are attempting to project that argument as one made by a third person.
I do not believe that sodomy between consenting adults should be illegal. I don't believe in outlawing pornography either. Adultery....hmmm, not sure about that one. Off the top of my head, I prefer huge penalties in the divorce settlement for that.