What behavioral and biological changes would be needed to make males not fight each other over females and for females to fight each other for males and to not make males expandle and extremely important for the species?
These are not weapons, for doing battle! How gauche. Nay, in this world brilliantly bedecked males dazzle the females (and each other!) with glorious feathers, fur tufts, multicolored appendages and eyes like limpid pools. And not just that. We showy spectacular males also croon with sexy voices as well as wail plaintively and do that rock and roll scream where I jump up and kick my legs! Did I mention I also play guitar? No; please - you are too kind. I am still just learning.
And ladies - get close. I will sing less loudly and I promise not to kick. If you brush up against my feathers it is ok. That is what they are for. Now - do you smell that? That is the great smell of Brut and it is coming from me. Oh, you with the red hair, you detect a little cinnamon as well? That is because I just baked cinnamon rolls! Would you like one? I'll split one with you. Dudes I am ok with you guys having some cinnamon rolls too. I made plenty so we could share.
Yes the men don't need to fight. We show off by being awesome in dozens of awesome ways as some men already do. The ladies are competitive but this does not require the world to change much.
One possible biological change would be to make male births more rare, as in 1/4 of the population or less.
It would not necessarily produce absolutely peaceful males but it should serve to greatly reduce the need for male-on-male aggression.
If the disparity is significantly greater you could even approach a point where the males cannot be risked because then you don't have a continuing population.
One less testosterone. Human males need and produce a large level of testosterone.
testosterone encorages competition. It also increases uperbody mass make men more suited for fighting. You would need to change them so that they need and produced less testosterone. This would make them weaker and less competitive .
Infertile females. Cant have your warrior class coming down with children while in the middle of a campaign. To get the exact results you want then you will also need an expendable class of females to send into battle. Simular to a bee hive. Have your females divied in to groups the hypersexual baby factories and the asexual soldiers/workers.
An intersting possibility would to create a species were females could switch between warrior form and breader form by undergoing a metamorphosis.