> Everyone says things in private they wouldn't want their employer to hear.
That's the difference in this case. The men in question were not in the personal life having a private discussion, they were attending a conference that their employer was a sponsor of and they were marked as being representatives of that sponsor company via their badges. They are already damaging their own and their company's reputation by making dumb sexist jokes at a professional-for-them event.
Until the day a man feels equally comfortable telling a sex-related joke to a woman as to another man, all such jokes are, by definition, sexist. We as a society are not open towards sex and the real sexist men feel that it is to their advantage that women should feel more embarrassed about sex jokes, even a woman have no reason whatsoever to feel that way.
You make a key point: the men were not in their personal lives having a private conversation. I also used your "professional-for-them" in another post.
That's the difference in this case. The men in question were not in the personal life having a private discussion, they were attending a conference that their employer was a sponsor of and they were marked as being representatives of that sponsor company via their badges. They are already damaging their own and their company's reputation by making dumb sexist jokes at a professional-for-them event.