It is certainly true some people went naked or near naked into battle, but not alongside warriors in proper armor and not in battle bikinis. There are some other important factors involved in their choice to do so. Firstly it usually only in cases where they didn’t have access to armor and/or the battles were largely ceremonial or otherwise non-lethal.
Armor is developed in response to weapons and usually the first forms of defense were shields. So if you had no nudity taboo and hadn’t developed armor due to lack of resources or lack of regular conflict, you didn’t really have much choice in the matter. Particularly since your weapons are usually tools that are made for hunting or other work.
In areas where this happened, usually the battles were no war in the sense of systematic killing of the enemy but more demonstrations of strength to intimidate others – usually over a piece of farm land or livestock. It was used to resolve grievances and sometimes even as a regular sport.
Usually this happened where people needed everyone to work together in order to provide essential, which means you also don’t need any more land than you already control and work every day. When you have an argument with your neighbours, you settle it to both sides satisfaction so you can resume living next to one another.
The ability to make sophisticated items like bikini armor (which is surprisingly complicated) comes from civilizations where they have sufficient surplus of resources and people they can have specialists who can trade goods and ideas. By the time you reach this level you also a real incentive to try to obtain more and more land.
At that point civilizations can start developing dedicated weapons, training dedicated soldiers (to expand your nation or defend against invaders) and their battles start to involve countless fatalities. Then it becomes worthwhile to begin the cycle of making armor to protect against the enemies weapons, and weapons to beat your enemies armor.
TL;DR: If you’re in a society that has warriors and the know-how and resources to make bikini armor, you’re in a society where your warriors wear actual armor. There were civilizations that fought nude or near nude, but they didn’t have bikini armor, fancy swords, professional warriors or sophisticated combat techniques.
– wincenworks