I need to connect a portable generator to my gas furnace blower. It seems that I can put male and female plugs between the manual on/off switch and when I need to use generator power just unplug the house feed and plug in the generator.
I don't have the bucks to hook up the whole house which would be the best way.

Are you in the US? Using a cord is a violation of the NEC 422.16.

They make a single circuit transfer switch for just such a purpose. It can mount anywhere in the line. When the power goes out you plug your extension cord into the switch and flip a switch. All NEC approved for about $80.

Amazon.com: Reliance Controls TF151W Easy/Tran Single-Circuit 15 Amp Furnace Generator Transfer Switch For Up To 1875 Watt Generators: Patio, Lawn & Garden

If your furnace has a wire that plugs into a receptacle. Just use a extintion cord to make the switch. If your system does not have a receptacle it plugs in to you will need to add it for the quick and easy change over.

The easiest way to get power to your furnace is to put power to your electrical panel. You can use transfer switches but they take a lot of wiring. Look up interlock kit on Google. Their website is pretty good. For $150 you can do it with a little afternoon work. The kit makes it legal and safe.