Good point. But not all of the anti-Franco forces were Loyalists/Communists, or were supported by the USSR. Remember the POUM which George Orwell fought alongside? They were anti-Soviet and anti-Franco, since they were anarchists. Anarchism would have been preferable to fascism or Soviet Socialism IMO, since its a more puritanical form of libertarianism. But of course the POUM were slaughtered by Franco and the NKVD, along with Spain's chances for democracy and freedom before Franco's death in '75.
You had the choice between the lesser of two evils in that war. Had I been Spanish, my preference would have been to leave. As an Englishman, my preference would be not to fight in that war at all.
Regards, Ivan