I always loved seahorses! From what I know both the males and females compete for mates, but the males are more aggressive while the females are more passive. I know that males sometimes fight other males - but they usually never cause any lasting damage. (From what I have read)
As for a male trying to fight a female, I don't know. I hope someone answers your question.
How long have you had seahorses?
Could be a stupid question, but are you sure that the males are males and the female is a female? Perhaps you sexed them wrong? (Like I said could be a potentially dumb question)