While it's not true that you have to use leverage to participate in Forex, the alternative makes it impractical for most people to be able to do so. You need to be able to put a lot of money into it in order to not trade on leverage.
The fact is, most accounts for "normal" people require leverage because the size of the typical contract is more than the average person can afford to risk (or usually more than the average person has).
Leverage, however, in the Forex market is not like Leverage in the stock or commodities market (well, they're the same thing in theory, but they are executed differently). In Forex, the broker is the one lending you the money in nearly all cases, and they will cash out your position when your account balance is exhausted. Thus, there is no risk for them (barring fraud or other illegal issues). Technically, I don't believe they guarantee that you will not accrue a debt, but I've never heard of anyone having their position cashed out and then owed more money. They've very good about making sure you can only spend money you've deposited.
To put this another way, if you have $1,000 in your account and you are leveraged to 100,000. Once your trade drops to $1000 in losses your position is automatically cashed out. There is no risk to the broker, and no risk to you (other than your $1000)... So trading without leverage has little value, while traiding with leverage has lots of potential gain and no downsides (other than a faster rate of loss, but if you're worried about that, just trade smaller lots.)