The problem, here, is the word "tax". Social Security is not a "tax" in that it is really only a forced retirement fund, for us. The money is taken from us while we have an income, and put away so that we can get it back, later, when we retire. It was not "taxed", per say, it was taken and set aside. Later, when we get it back, it's still "income" and is therefor still taxable as income. Think of it as a sort of forced 401K. It is a tax in that the government is forcibly taking money from us, but it's not a tax in that we get the money back, later.
They are not "borrowing" the money because their "I.O.U.s" really mean "You Owe You", since the government can only pay something back with our money. So they steal our retirement money, and then if they ever do pay it back, they will do so with yet more of our money. And we elect these dirt bags! Over and over and over, again!
And if this isn't insulting enough, the republicans are now trying to use the insolvency of the fund, THAT THEY CAUSED by stealing the money, as an excuse for discontinuing the Social Security program! And if they succeed, they will steal the rest of the money in it, and then leave millions of citizens without any retirement money at all (to go along with the health insurance millions of us won't have, because the republicans have also defunded Medicaid to give big tax cuts to themselves and their wealthy sponsors).
What wonderful, thoughtful, leaders we have running our government!