This is not correct. It is explicitly stated in the novel that the government has to accept anyone who volunteers. Someone who conscientiously objects to military service will be given something else to do, but as long as they volunteer for some kind of service, they can't be denied.

> being in military does not make you responsible

Depends on your definition of "responsible". In the absence of mind-reading ability, we have to assess that indirectly. The indirect method Heinlein portrays in the novel is the willingness to sacrifice one's life for one's country. Doesn't seem like a bad indirect method to me.

> the only way to gain the vote is to subject yourself to years of pro-military propaganda in environment where dissent is treated as undiscipline and where the tactic to make you adopt new values literally includes sleep deprivation and maximum exhaustion

The only training explicitly portrayed in the novel is that of the Mobile Infantry; the training is driven by the requirements for the MI to be effective at its mission, not by "propaganda". And most people who volunteer don't end up in the MI (not stated explicitly IIRC, but certainly strongly implied by much indirect evidence). And it doesn't seem like the training that Navy pilots, for example, go through is anything like MI training (again, not stated explicitly but strongly implied by indirect evidence).

> military dictatorships and dictatorships in real world did not proved to lead to more responsible exertion of their sovereign authority.

The government of the Terran Federation is not a military dictatorship. It is specifically described as a representative democracy in which you have to volunteer for 2 years of service to your country in order to vote or hold office. What's more, you're not eligible to vote or hold office until you complete your service, which means that if you are a career military person, you don't get to vote or hold office until you end your career. So, for example, the Fleet Seargeant who gives Rico and his friends the long speech before swearing them in, and the Colonel (temporarily reduced from Fleet General) who gives Rico his pips before Rico goes out on his field training as an officer, cannot yet vote or hold office. They won't be able to until they retire.