Due to the nature of fighter planes - their size and the lack of stops for bathroom breaks - pilots will often wear or have access to forms of diapers. Female fighter pilots are more likely to wear diapers whereas male fighter pilots will sometimes use a piddle pack.
The reason female fighter pilots are more likely to wear diapers is due to the way they use the toilet. Where male fighter pilots can aim into a piddle pack funnel, women often find this a difficult and time-consuming task when they have bigger problems to face. As diapers are a much more convenient way of going to the bathroom than anything else, most male fighter pilots will also wear diapers.
Piddle packs are less convenient and more awkward than diapers. These are plastic packs that require partial undressing which converts the urine into a gel consistency, which is easier to dispose of - otherwise, you’re dealing with the potentially messy business of urine everywhere in the plane.
When it comes to convenience, diapers are commonly used by fighter pilots if they don’t feel comfortable using piddle packs or other methods of urination.