You will be surprised by the actual answer, but those ladies have no medical problems whatsoever by wearing very short skirts and thongs or no undies at all.
In fact, it’s the other way around.
First of all, do not forget that we all sit on toilet seats (wherever they are) on a regular basis for our entire life when No. 1 or No. 2 require it, and virtually none of use ever suffer from medical consequences — even if thousands of people have seated there before us, and even it their bottom sides weren’t clean. Our bottoms can handle it just fine.
Secondly: those among us who actually fear the bare contact with a surface where someone’s bottom has been just minutes before and therefore hover over the toilet seat, actually often do have to deal with medical problems afterwards.
Hovering obstructs the urinary bladder from emptying in the right way, thus leaving residual bacteria and subsequently often leading to Urinary Tract Infections (UTIs). (To add insult to injury, hovering also causes difficulties in relaxing the pelvic floor muscles — which makes matters even worse.)
(One patient of my girlfriend (who is a urologist) was convinced she had bladder cancer because she had UTIs on a regular basis. She didn’t have cancer, though — she systematically hovered.)
So actually there are no hygiene problems at all with touching those surfaces with out backsides —
It’s those that strive not to touch them that get germy.