Favors, like sex, are also a means of exchange. What is legally the difference between taxing virtual currency and taxing sex?
First of all, what do you mean by "taxing virtual currency"? Are you talking about the sales tax? VAT? Capital gains tax, and capital losses credits?
By the way, there is nothing virtual about Bitcoin. It's a hard currency. Irreversible.
Next, which jurisdiction are you considering?
Finally: I just paid for massage. The goods-and-services tax was included.