Some period pants are not treated with stain resistant chemicals because they can be damaging for marine life when the pants are washed.
Most period pants are black so even if they are slightly stained it won’t be visible. That being said, some people prefer light-coloured products because it can be easier to tell when the pants needs to be changed.
If you rinse your pants with cold water as soon as possible after use you can reduce the chances of staining.
If staining does occur you can spot-treat them using this natural product.