Naomi - //How serious the problem is depends upon what one perceives to be sexual harassment. The picture in the link shows a woman with a man's hand on her shoulder. I wouldn't necessarily consider that to be sexual harassment - but some would. //
That is the root of the issue.
Working with other people is about looking and listening and interpreting what you see and hear.
If a colleague clearly enjoys banter and gives as good as he or she gets, then you can probably indulge in some risqué exchanges.
If another colleage is clearly shy and doesn't join in, then they shouldn't be the butt of ribaldry.
That should steer people through the potential to offend unintentionally, which should be addressed immediately with an appropriate and sincere apology.
When someone is conducting a campaign of harassment, then that is an entirely different matter, and should be dealt with immediately and send a message that such behaviour is not tolerated.