Underwear offers a barrier that keeps residual urine and feces particals from absorbing directly into your pants.
People that dont wear underwear let their bodily fluids make direct contact with their pants, and then expose them directly to any surfaces their bits might touch.
Not wearing underwear is unhygienic.
Eh, I don’t think it’s quite that serious lol. People become germaphobic for a ton of different reasons. I still have a really heavy impulse to wash my hands after touching surfaces due to covid, for instance.
Severe or unexpected illnesses experienced by a person or seen in their family can cause a lot of trauma. With the US healthcare and work system being as awful as it is, it’s not really irrational to be driven to being overly cautious. I think that as long as people aren’t having anxiety problems or compulsions that impact their quality of life, being a little too cautious like OP is pretty benign.