It's (in the US) constitutionally allowed, which is analogous to "we agree to be taxed to support our government". It's true that you may not have personally agreed to it, but that doesn't really scale anyway. The constitution 'stands for' our mutual agreement - which is why we're allowed to change it.
Even if you don't like that argument, you have to appreciate that it works for many people and so you won't get much of anywhere declaring "taxation is theft" like that's obvious and unquestionable; you just lose 98% of everyone right there. (okay, maybe 95 or 90. I don't know anymore.)
A different argument: In an anarchy, taxation would be theft. In our system of government (being US-centric again), it is not. You can argue that you'd rather live in an anarchy/minarchy/whatever, and that's fine, but it's silly to declare that taxation is theft universally because in the current system that's not really true.