Final answer:
Swift rejects the idea of eating teenagers in 'A Modest Proposal' because their meat would be tough and lean, the cost to fatten them would not be worthwhile, and consuming females would eliminate potential future breeders, not to mention the border on cruelty which he strongly opposes.
Step-by-step explanation:
In Jonathan Swift's satirical essay A Modest Proposal, he rejects the idea of eating teenagers for several reasons. Firstly, he argues that the flesh of teenage males would be tough and lean, unfavorably compared to schoolboys who engage in frequent exercise which makes their meat less desirable. Additionally, fattening them would not be cost-effective. Secondly, for the females, he believes that eating them would be a loss to the public because they would soon become breeders themselves, contributing to population growth. Moreover, he notes that this practice might border on cruelty, which is a strong objection for him against any project, regardless of the intentions behind it.