If con is the opposite of pro...
If con is the opposite of pro, then isn’t Congress the opposite of progress?
Welcome to day 2 of political jokes.
This joke is based on a false etymology of the word Congress. The word congress actually comes from Latin, and means "to walk together", and "con" is the part that means "together". When we say "con" (as in "pros vs cons"), it is short for the Latin word "contra", which means "against" (whereas "pro" means "for, or on behalf of").
The word Congress does have "con" in it, however the root of the word does not make it mean the same thing as the word con. The joke is poking fun at Congress, and saying that they never get anything done, and using a false etymology to do that.