O filósofo católico Edward Feser escreveu um excelente texto sobre as condições para que um papa declare algo infalível e estas condições deixam claro a razão de que um papa não pode, nem que queira, aprovar coisas como ordenação de mulheres ou casamento gay.
Ele explica por que um papa não é idêntico a um monarca absolutista, que pode definir tudo apenas de sua cabeça.
Feser traz também uma excelente descrição dos casos históricos.
É um texto para ser guardado, o ilustre canonista Ed Peters disse que vai usá-lo em suas aulas.
Vou colocar aqui apenas o início do texto de Feser, leiam todo no site dele, clicando no link.
by Edward Feser
Catholic doctrine on the teaching authority of the pope is pretty clear, but lots of people badly misunderstand it. A non-Catholic friend of mine recently asked me whether the pope could in theory reverse the Church’s teaching about homosexuality. Said my friend: “He could just make an ex cathedra declaration to that effect, couldn’t he?” Well, no, he couldn’t. That is simply not at all how it works. Some people think that Catholic teaching is that a pope is infallible not only when making ex cathedra declarations, but in everything he does and says. That is also simply not the case. Catholic doctrine allows that popes can make grave mistakes, even mistakes that touch on doctrinal matters in certain ways.
Many Catholics know all this, but they often misunderstand papal authority in yet other ways. Some think that a Catholic is obliged to accept the teaching of a pope only when that teaching is put forward by him as infallible. That too is not the case. Contrary to this “minimalist” view, there is much that Catholics have to assent to even though it is not put forward as infallible. Others think that a Catholic is obliged to agree more or less with every view or decision of a pope regarding matters of theology, philosophy, politics, etc. even when it is not put forward as infallible. And that too is not the case. Contrary to this “maximalist” view, there is much to which a Catholic need give only respectful consideration, but not necessarily assent. As always, Catholic doctrine is balanced, a mean between extremes -- in this case, between these minimalist and maximalist extremes. But it is also nuanced, and to understand it we need to make some distinctions that are too often ignored.