What does david copperfield's joke mean?

I introduced my girlfriend to my buddy and thought of the quarrel with my girlfriend yesterday.

I accidentally said, this is XX, my temporary girlfriend.

I didn't feel right just after I finished, and my buddy was shocked on the spot.

Look at your girlfriend again and glare! The situation is very critical!

quickly add: because it will be my wife in the future!

the tense air suddenly eased.

I've been living by this reaction all my life. Damn it, I'm scared to death!