When property rights are clearly defined, the parties who exchange must compensate those who would be damaged by their production that are not parties to the transaction. The parties exchanging often buy insurance that will compensate for any unintended damages. Thus, in one way or another, the social costs are internalized. One way has been outlined by Coase, et al. In sum, proactive public policy is thought to be able to improve on what the market produces or to alleviate the market's failure.
Abortion involves a market failure.
The transactions costs are very high for him to bargain because he cannot talk yet or write or bargain in any way. Yet this fact does not necessarily empower public policy to provide more optimal results than Coasean bargaining. The unborn child is a minor and, therefore, just a 5 year old minor who is damaged by soot from a factory, someone else from the human species may vouch for him as guardian and vicar. Plus, it is well established practice in law that this benefactor need not be his natural parents. Indeed, classical liberalism would have no qualms with anyone entering on a child's behalf in order to save or protect him.