Ben Hart answers:
When horses begin to behave in these unwanted ways there are two important things to remember as a starting point.
First, this behaviour is normal for horses and only becomes a problem because of the restricted environments in which we keep them.
Second, your horse’s behaviour is telling you something about how he feels and his emotional state. Looking at his behaviour in this way makes it easier to search for the causes and to recognise his distress rather than blaming him for the inconvenience he’s causing. It sounds as though your gelding wants to be with this mare and, to him, she’s a valuable resource worth protecting from other horses, but it’s causing stress to him – and to you – and obviously presents a risk of injury to his fieldmates, and possibly their handlers.