Most modern ideas about grossness is just natural self defense mechanisms to prevent disease spreading. Sharing toothbrushes is a highly effective way to transfer bacteria, more so than kissing (unless you use your tongue to really get in there between the teeth...)
For heaven’s sake, I’m okay with us rubbing our naked bodies together to produce these little people, why would I not be okay sharing a towel?
People are weird. Kissing is fine but I can't use your toothbrush? Oral is fine but you want me to wash my hands after I go to the bathroom?
Why does everyone have to be so picky?
Center for Disease Control
"Teaching people about handwashing helps them and their communities stay healthy. Handwashing education in the community:
Reduces the number of people who get sick with diarrhea by 31% 1, 2
Reduces diarrheal illness in people with weakened immune systems by 58% 3
Reduces respiratory illnesses, like colds, in the general population by 16-21% 2, 4"