What you said
The child's comment was upsetting. I wonder why you told him his behaviour had not improved much since Year 3. The child may have been hitting back at what he saw as a negative character evaluation. Talk to your line manager about what happened and why. Stepping back and reviewing the situation may give a different perspective.
You were slandered at work and your employers have a duty of care to you under the Health and Safety at Work Act 1974. Perhaps your union rep could remind the school of that. From now on, log and report via email to the assistant head everything of this nature.
The expert view
This is bizarre. From the details you have given, it is unclear why you are expected to apologise. It was an inappropriate and sexual slur to make about an adult. You are not obliged to accept an apology.
It is poignant how pupils are often happy to be rude in public but apologise only in private - and that this boy insulted you publicly but you appear to be facing pressure from management for also making your feelings clear publicly.
Without knowing more, it is impossible to comment properly, but: no one can make you apologise, and if it is not a genuine apology there is no point. You need representation before any meeting takes place on this issue, because you must not be strong-armed into a situation you are not happy with. Good luck.
Tom Bennett's latest book, Teacher, is out now, published by Continuum. Post your questions at www.tesconnect.combehaviour.