Speaking from experience, they might not be your problem.
Visit a doctor if you have serious problem
1: Tight panties: If your panties are too tight, I mean extremely tight and your boxers for the male, it makes you itch a lot and cause bumps. Because no ventilation?
2:Wash Up: After using the toilet wash up, with water most time tissue papers don’t take everything off some some of the particles from your stomach remain.
3:Avoid same underwear: I mean avoid wearing one underwear for multiple days yes it causes you to itch
4: Clean Up after sex: oh yes do it, some people just dress up after having sex without cleaning up there private parts . You need to clean up.
5:Avoid public toliets: I mean dirty ones , in case you are not home A lot of germs are there, try as much you can to avoid it.