Taxation is in some sense unethical--it is a 'taking'. But that doesn't mean the alternative is perfectly ethical. And I'm not talking about the fact that non-taxation could lead to poverty, social distress, and other ills that would be 'worse' than taxation (both pragmatically and ethically). There are arguments to be made there, but at present I want to instead focus on another idea: money is non-linear.
Money is our book-keeping method to avoid tedious micro-bartering all the time. It is a way for us to assign value to all the things we care about (from land, to resources, to time, to talent, ...). To a first approximation, we can treat it as a linear object (work twice as long, get twice as much pay; want 2 apples instead of 1, you pay twice as much). And yet we all know that there are many non-linearities in the money system. Everything from economies of scale (actually it's cheaper to buy in bulk), to 'rich get richer' (concentrations of money can actually be more powerful than the sum of the individual quantities of money), to psychological factors (happiness as a function of wealth, .99 pricing, the low value assigned to loose change, ...), etc. The way people value money is also non-uniform (the 'value' of $100 to people living in different areas is not the same) and is non-linear: as you become richer the marginal value of each new dollar is in some sense lower (hence why a rich person can basically value a dollar less than a poor person). Money is also complicated in other ways, such as its value changing with time (inflation) and format (liquidity), money requiring stable infrastructure to retain value (hence its value is contingent), and so on. These are just example; I'm sure you can think of more.
The point is that the value(money) graph is not necessarily linear. It's not even necessarily monotonically increasing. It's complicated, and really we don't know what it 'truly' looks like. (Even defining the question is hard since it varies from person to person, from place to place, and whether we're talking about 'purchasing power', or 'utility', or 'value', or 'happiness', etc.) Money is a crude approximation for what we really care about: value. (Yes, part of my argument is that morality/fairness/etc. should really be concerned with value (how much things matter to us) and not the artificial construct of money.)
Imagine we knew what the 'right' value(money) curve really was. We could then imagine all transactions occurring in 'new money' which is a linearized form of 'current money' where a myriad of calculations go on in the background to make 'new money' linear in terms of value(money). This would be horrendously complicated (possibly impossible, due to various inhomogeneities), so it's obvious why we don't do that: we just stick to our simplistic and naive linear-money model.
This brings up two points:
1. In some sense, you can think of many aspects of our current money system as being crude attempts to tweak the current naive money model so that it makes more sense. Things like interest rates are tweaks that somewhat offset inflation. Taxation is a tweak that takes into account marginal utility and externalities that were required to give that money value in the first place. We can certainly debate about what tax system would provide the fairest 're-numbering' of the money system... but it's by no means obvious that no taxation would really be the most ethical numbering. (Again, imagine two identical worlds, one using current money, and one using the hypothetical complicated 'new money' presented above... A tax in one numbering system appears as no tax in the other. Which one is 'right'? Well, it depends whether you consider linear money 'right', or whether you think that money is non-linear.)
2. My main point, however, is that arguing that taxation is theft is unfair. Because money is so complicated (historical, contingent, non-linear, inhomogeneous, chaotic, prone to feedbacks and emergent phenomena, etc.), it's by no means obvious which model is really the most 'fair' or 'ethical'. Simply not auto-decrementing your account balance is not necessarily fair (e.g. the government could just 'tax' you by printing more money, thereby decreasing the value of what you have, which is indeed one aspect of how we're all taxed).
Again, to be crystal-clear: I'm not saying that the current tax system is necessarily optimal, fair, or ethical. I'm saying that, abstractly, non-taxation is not necessarily fair. To know whether (or how much) taxation is ethical, you need to look in detail at the monetary system in place. And non-linearities make this complicated. The naive assumption that 'no taxation == no injustice' and 'taxation == theft' relies on a naive assumption that our current linear money model is correct. I challenge that assumption.