When alcohol is introduced it's easy to see how lines can get crossed.
It is the employers job to make sure everyone knows the expected behavior and the guidelines, without this some can get too relaxed and take things too far and become inappropriate. This could range from Monday morning gossip through to disciplinary procedures needing to happen. Many employers give their staff some leeway at the Christmas party but as an employee you need to make sure you know the boundaries.
As an employer set some rules in advance and make sure everyone knows these to ensure safety and enjoyment. Think about providing food, transport options and a plan of the event to ensure a smooth outcome. You need to lead by example and keep an eye out for anyone who is edging towards crossing the line. You might need to tactfully remove people from a situation or conversation. The safety and well-being of your team is your responsibility and if an employee is feeling uncomfortable, unsafe or unwell it is your responsibility to do something about it.
If last years Christmas party became a little too rowdy or inappropriate maybe think about changing it from an evening party to a day time activity or a family event.