Judging by your tag, you're a male. As a female who is relatively small and works in a vastly male-dominated work environment, I'd just like to point out that yes, those sorts of threats (whether at work or not), do create a hostile work environment.
Let me give you an example.
If I, say, was working with a man who had committed the same offenses but WASN'T fired, I would feel genuinely uncomfortable being alone with him at the office. I would feel that I didn't want to be in a room with him. I would spend hours agonizing over whether or not my bosses agreed with him and thought that these "drunken mistakes", while "accidental" (according to your own theory that it's a one-time drunken offense), and that would ruin that workplace for me.
At the end of the day, the company did the right thing. They stood on the side of people who constantly face this sort of harassment (often women, though not always), and they showed that either in or out of the workplace, they will not tolerate people who espouse such behavior.
By your own theory that private and public lives should be separate, a senator who votes against gay marriage but has gay affairs should be unaffected by his personal life, right? A member of the Ku Klux Klan should be able to keep his job at a mixed-race firm even after racist rants come to life? (These are real examples, by the way).
At the end of the day, you pay for your mistakes. Any progressive workplace is going to say that your private life is your private life...if you make a reasonable attempt to actually keep it private (such as using a site like E, where the vast majority of boards are secret except to approved users). If you go shoving that 'private' life into the face of the public, then you're stepping into the bounds of what reflects upon the company. If you think that the actions of one employee can't taint the image of an overall company, you're mistaken.
Additionally, most American companies require employees to sign codes of conduct with their contracts that state that they will not do certain things - generally including heavy drug usage, engaging in sexism, etc. - both in and out of the workplace.
I think he got what he deserved and I applaud the company for thinking of the people who would have to work with him rather than falling on his side. He did something stupid and he got punished. Companies, particularly privately-owned ones (as well as public ones), have the right to decide who is working for them. They choose employees that fit their image and their ideals. If an employee violates that, they deserve to be sacked.
Also, for everyone using the slippery slope argument...I would love to see some examples of companies abusing this policy. The VAST majority of cases (as in, for every 1 poor example there are thousands of legitimate examples like this) of firings relating to poor outside-of-work behavior have to do with cases like this. When a man makes a joke like that, it is not a joke. Anyone who thinks it is has never genuinely felt the fear or threat of being smaller or in a situation where rape was a legitimate threat. If someone says "this doesn't feel like a joke", that is something that should be respected. To say that "I don't see it that way, so people should be free to make that sort of joke even though it's not funny and they shouldn't be punished in any way" (a general breakdown of what I've seen, since your version of punishing seems to be purely personal and in no way financial (aka job related)" is not only somewhat insulting, it is genuinely scary. It is scary to realize that a number of men take these things as casual jokes.
For every ten men that make a joke like this, there will be one who is genuinely considering those actions (obviously this is an example and not a statistical number). While free speech in America is a right, so is the right to take a stand against those who make "jokes" that are not only threatening, but undermine women in the workplace.
TL;DR: He is free to say whatever he wants to say...just as the company is free to say that they find his behavior disgusting and, like a friend who has abused your trust, they will no longer associate themselves with him.