If you wear boxers that could definitly be you problem trust me I was hiking and sweating all over the place and got a rash that I really don't want to describe while wearing boxers they actually can cause a lot of problems its thermodynamics at work switching to briefs will help if you sweat a lot. also there are light wieght nylon pants that look like khakis that are extremly comfortable get a loose fitting pair and wear briefs and you'll be good hopefully

i think other material (natural) pants would do the work!