Isn't that a part of the attraction of bitcoin? You can AVOID tax? And as for "realized gains", why not just sell your bitcoins offline to some person willing to show up at a coffee shop and pay cash?
No, the the attraction of Bitcoins is fast global payments an order of magnitude cheaper than anything else.
Of course you should pay taxes if your investment pays off. It's income just like anything else and that's what makes society work. If I had lived outside society I would statistically not have had the opportunity to learn about this stuff, let alone own a computer.
What you say about cash transactions is true for all foreign exchange speculation. Sure you can trade USD and JPY by meeting random people on the street from the classified section and exchange cash. Then all your speculation would be outside taxation. But if that seems like a worthwhile endevour for you, you are investing way too little!