Without taking a position on Catalonia, I think one has to analyze the reasons they want to hold the referendum and decide if you find them compelling. When the USA seceded from the UK, the Declaration of Independence provided a reasonable list of reasons. By contrast, the main reason the CSA seceded from the USA - slavery - prevented their cause from being legitimate.
Independentism is always a thorny subject, because most justification ends up being defined ex post. It’s like “resistance” movements against lawful but tyrannical governments: if they succeed they will be freedom fighters, if they fail they’ll be brigands and terrorists. Usually the threshold for legitimacy is overwhelming support from the population, something that happens sporadically (it happened in Slovakia, Slovenia, Croatia, Kossovo, almost in Scotland, and of course in post-soviet republics all over the old federation). But once popular support is somehow made manifest, then the Wilson doctrine must be allowed to set the agenda.
Invading a country to abolish slavery is morally justifiable, invading a country because you want to tax them isn't.
Who gets to decide if the Principality of Hutt River (75 square kilometers, so larger than San Marino) can secede from Australia?
Only those who live there? Or can other Australians be involved?
This is one reason why trying to deal with secessionists in a democracy is so difficult (see also Canada, and Ireland).
When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final.
Similarly, Catalonians and the rest of Spaniards approved their Constitution in 1978, which states that matters like this shall be voted in national referendum, once approved by congress.
Secession is by definition a break out from the collective. So considering what the collective wants seems to me like asking the majority to vote for their tyrannical interest (very biased way of doing things).