#2 is almost a "legal loophole". Both versions often are meant to be the same thing, but #2 leaves the possibility that they can "squeeze through the loophole" and get it done by someone else, and not be lying to who they are talking to.
#2 is also more definite. It's the sort of thing a politician would say while pounding the desk - but in that case then they almost certainly will get someone else to do it!
In short it depends on who is saying the words. If it's a politician or an equally careful individual, I'd say they meant #2a. Otherwise almost certainly #2b. Of course, if it turns out that they do #2a, then that puts them in the category of "politician"!
Understand first that both "go to work" and "get to work" have multiple meanings. Both can refer to the process of traveling to your place of work. Both also mean "start working".
In the context of traveling, the difference is similar to the English use of "go" vs. "come". If I say I need to go to work then I picture myself moving toward my place of work. If instead I say I need to get to work then I picture myself already at work and the fact that I am not actually where I should be.
In terms of the second meaning, the difference is similarly subtle. "Go to work" is used when starting a project, while "get to work" can be used anytime to mean either "start work" or "work harder".
Which to use can vary with context:
All right team. We've prepared everything, now let's go to work building it!
Steve, stop chatting with Mary and get to work.
We're already way behind on this project; we need to get to work if we're going to finish it on time.
The team of lawyers went to work on the contract to see if they could find any grounds for a breach.
Note without any additional context, simply using "went to work" instead of "got to work" implies the process had just started. I could instead say:
The team of lawyers got to work on the contract ...
which would not necessarily change the meaning, but it also would not as strongly imply that it was just the start of the process.
In the example with your coworker, since it's assumed that he was already working and stopped to chat with you, then "get to work" is correct. Although, it probably would have been even more appropriate to say "get back to work":
Steven I'm really busy. Could you stop distracting me and get back to work? Thanks.
Best Answer
Your guess is correct, and while other answers have already confirmed this, I just wanted to share the "missing parts" of the sentence that might help you gain a better understanding:
The speaker, a male, makes the joke about the fact that (in his opinion) breasts are distracting (from the sexual viewpoint of a male), to the point of not being able to do anything else but looking at them.
However, one of the key points of the joke is that the speaker is not taking into account the fact that women do not feel the same way about their own anatomy.