Is it obligatory to wash the thighs if they are splattered with a spray of feces when relieving oneself in Western toilets, and how can this be avoided, and is it obligatory to wash the trousers if they become wet from the underwear after washing the thighs?

If a person is certain that some water from a toilet contaminated with urine or faeces has splashed onto their body, they must wash it off. If they put on their clothes before removing the impurity, they must take them off and wash off the impurity. They are not required to wash their clothes unless they are certain that impurity has touched them, in which case they should wash the affected area. Otherwise, the default is that they are pure. The individual is most knowledgeable about how to prevent contaminated water from splashing, depending on the toilet and its condition. If they are unable to prevent it, the matter is easy because they are commanded to purify themselves if they are certain that impurity has touched them.