Every one has a right to live. We will suppose this granted. But no one has a right to bring creatures into life, to be supported by other people. Whoever means to stand upon the first of these rights must renounce all pretension to the last. If a man cannot support even himself unless others help him, those others are entitled to say that they do not also undertake the support of any offspring which it is physically possible for him to summon into the world. Yet there are abundance of writers and public speakers, including many of most ostentatious pretensions to high feeling, whose views of life are so truly brutish, that they see hardship in preventing paupers from breeding hereditary paupers in the workhouse itself. Posterity will one day ask with astonishment, what sort of people it could be among whom such preachers could find proselytes.
It would be possible for the state to guarantee employment at ample wages to all who are born. But if it does this, it is bound in self-protection, and for the sake of every purpose for which government exists, to provide that no person shall be born without its consent. If the ordinary and spontaneous motives to self-restraint are removed, others must be substituted. Restrictions on marriage, at least equivalent to those existing  in some of the German states, or severe penalties on those who have children when unable to support them, would then be indispensable. Society can feed the necessitous, if it takes their multiplication under its control; or (if destitute of all moral feeling for the wretched offspring) it can leave the last to their discretion, abandoning the first to their own care. But it cannot with impunity take the feeding upon itself, and leave the multiplying free.
To give profusely to the people, whether under the name of charity or of employment, without placing them under such influences that prudential motives shall act powerfully upon them, is to lavish the means of benefiting mankind, without attaining the object. Leave the people in a situation in which their condition manifestly depends upon their numbers, and the greatest permanent benefit may be derived from any sacrifice made to improve the physical well-being of the present generation, and raise, by that means, the habits of their children. But remove the regulation of their wages from their own control; guarantee to them a certain payment, either by law, or by the feeling of the community; and no amount of comfort that you can give them will make either them or their descendants look to their own self-restraint as the proper means of preserving them in that state. You will only make them indignantly claim the continuance of your guarantee, to themselves and their full complement of possible posterity.
On these grounds some writers have altogether condemned the English poor-law, and any system of relief to the able-bodied, at least when uncombined with systematic legal precautions against over-population. The famous Act of the 43rd of Elizabeth undertook, on the part of the public, to provide work and wages for all the destitute able-bodied: and there is little doubt that if the intent of that Act had been fully carried out, and no means had been adopted by the administrators of relief to neutralize its natural tendencies, the poor-rate would by this time have absorbed the whole net produce of the land and labour of the country. It is not at all surprising, therefore, that Mr. Malthus and others should at first have concluded against all poor-laws whatever. It required much experience, and careful examination of different modes of poor-law management, to give assurance that the admission of an absolute right to be supported at the cost of other people, could exist in law and in fact, without fatally relaxing the springs of industry and the restraints of prudence. This, however, was fully substantiated, by the investigations of the original Poor Law Commissioners. Hostile as they are unjustly accused of being to the principle of legal relief, they are the first who fully proved the compatibility of any Poor Law, in which a right to relief was recognised, with the permanent interests of the labouring class and of posterity. By a collection of facts, experimentally ascertained in parishes scattered throughout England, it was shown that the guarantee of support could be freed from its injurious effects upon the minds and habits of the people, if the relief, though ample in respect to necessaries, was accompanied with conditions which they disliked, consisting of some restraints on their freedom, and the privation of some indulgences. Under this proviso, it may be regarded as irrevocably established, that the fate of no member of the community needs be abandoned to chance; that society can and therefore ought to insure every individual belonging to it against the extreme of want; that the condition even of those who are unable to find their own support, needs not be one of physical suffering, or the dread of it, but only of restricted indulgence, and enforced rigidity of discipline. This is surely something gained for humanity, important in itself, and still more so as a step to something beyond; and humanity has no worse enemies than those who lend themselves, either knowingly or unintentionally, to bring odium on this law, or on the principles in which it originated.
—Book II: Distribution; Chapter XII: Of Popular Remedies For Low Wages. Bolded emphasis mine.
This reasoning is extremely similar to that employed by Bryan Caplan in explaining why behavioral economics might mean we need to give the poor fewer options for their own good. Despite this similarity, Caplan's thinking on births is utterly opposed to Mill's.