Libertarians lose me whenever they use this argument.
According to the Militia Law all males between 17 and 45 are members of the Militia, whether organized (National Guard, etc) or unorganized (eg., the rest of us), therefore a draft is merely the calling up of the unorganized militia.
There is no BS about slavery, sir. It is the law and has been since pretty close to the beginning of the Republic.
Believe me, I think everyone ought to serve, as I did (2 years active, 6 reserve). What I object to, conceptually, is coerced service. The society whose members are unwilling to defend it does not deserve to stand against its enemies. That's why I like the idea of using military or equivilent national service as a qualification for full citizenship rights.
Your other post re Jefferson, Adams and Franklin is not apposite. In their time, there were other limitations on the franchise that ensured that only those with a significant stake in society voted. Each of them, as signers of the Declaration of Independence, placed their lives and property in peril for the sake of the country every bit as surely as if they had led regiments in the Continental Line. The point of using military or other service as a qualification is to ensure that each person who votes has demonstrated his or her willingness to place his or her life and property on the line for the country.