This is sexual harassment and is against the law. Your boss could well find his own job at risk – and rightly so – if you decide to take it further.
The pass itself is likely to be sexual harassment because it violated your dignity. Since it took place at a Christmas Party organised by your work, your employer is likely to be liable for this behaviour.
But to make matters worse, your boss has threatened to end your promotion chances if you complain. This threat is also an act of unlawful sexual harassment. This is because under the Equality Act, it is unlawful to treat someone less favourably at work because they rejected or submitted to unwelcome sexual conduct, such as a pass. It makes no difference where the pass took place.
Make a careful record of exactly what happened, talk to a trusted friend and get support to help you decide what is the best next step for you. If you are a union member, speak to your union rep.
If you are not a union member, you can get advice from the Equality and Human Rights Commission, Acas or Citizens Advice.
Your employer is likely to have clear rules banning sexual harassment and may also have a confidential reporting procedure in place that you can use if you decide to take the matter further.
In 2016, the TUC collaborated with the Everyday Sexism project to produce important research – Just a bit of banter? – revealing alarming levels of sexist behaviour in UK workplaces. Nearly a quarter (23%) of the women surveyed reported unwanted touching. In nearly 90% of cases, the perpetrator was male, and in around 20% of cases, the harasser was a line manager or someone with direct authority over them. So you are not alone, and it is not acceptable.