Why is there not a universal, natural right to citizenship?
1. What is required for one to be able to order the community, or legislate, well? [Civic] prudence and the moral virtues.
2. What about being able to judge other people well, according to their character (rather than the platform that they espouse)? This requires experience, familiarity with virtue, and right opinion regarding moral precepts and the good, at least, if not knowledge.
These are not inborn traits; they must be acquired and they are contingent, not necessary. As a result, there can be no natural right since no one is "naturally" qualified to rule or to have some share in it. Some notion of [natural] individual dominion or sovereignty is inadequate to back up the claim that such a right exists. In accordance with distributive justice, then, the work of the whole should be done by all the members of whole only if they are qualified to do it, not simply because they are members.
One might appeal to Aquinas' discussion of law as a counter-argument in favor of all being naturally entitled to citizenship:
Now to order anything to the common good, belongs either to the whole people, or to someone who is the viceregent of the whole people. And therefore the making of a law belongs either to the whole people or to a public personage who has care of the whole people: since in all other matters the directing of anything to the end concerns him to whom the end belongs.The act of legislating belongs to the community as a whole in directing itself to its good - "since in all other matters the directing of anything to the end concerns him to whom the end belongs." This could be further explained by an appeal to dominion or sovereignty (though Aquinas does not do so in the Summa Theologiae).
But not all participate in the ordering in the right way - some order, some are ordered. And even those who order may not do so equally, for example the members of the family. While the wife (and to a lesser extent their children) may contribute to the deliberation of the household, the husband is the authority. I do not think Aquinas would dispute this point. There appears to be a significant difference between an individual directing himself and an individual being a part of a group. The differences between the members and the consequences of these on their relationships to one another (~social dynamics) are such that the group is not reducible to the totality of the individuals who comprise it. The act of legislation may be attributable to the community as a whole, but it is not necessary that all members of the community are equally the author of that act - there may be justified degrees of participation. While it may be proper that all members of a community be consulted concerning potential legislation (especially regarding its impact on the community), this does not entail that all should be legislators.
While I am sympathetic to the claim that the best constitution (or the one most amenable to Christian fraternity) is that of a republic, it is an ideal and not one that comes about naturally. The paideia that is required must be in place in order to bring a republic into being and to sustain it. Nowadays, it is not as important question as the problem of size and scale, a more pressing matter, not only because of the lack of ecological sustainability.
If it is not in accord with distributive justice that those who do not have the requisite virtue have a share in rule, and it is not unjust to prevent them from having it, could a valid consequentialist argument be made that if they were to have a share in rule that a worse state of affairs would result (e.g. there would be more disagreement and conflict, or the quality of the legislation would decrease as more compromises would have to be made to satisfy the demands of opposing factions)?
An additional question that needs to be addressed is whether men and women are equally suited to rule. But perhaps a more important consideration is the relationship of a man to his family as the husband and father and to other men as a member of a group.