Generator Power without transfer switch

[buckbrush]

We have already had a few power outages lately. Fire place keeps the cold off for now. With the weather cooling down I want to use a generator.

I am thinking about wiring my furnace and water pump to plug into an extension cord. I am not talking about back feeding or "suicide cords".
I am wondering about Taking the existing wire and ending it with a regular wall outlet. Then taking some some extension cord wire and wiring in the furnace/ water pump to be like any other plug in appliance.

The furnace is a Natural gas Lenox, I have not checked the power draw on it yet.

Any reason not to get this done?