system
A chat on subreddit /r/NoStupidQuestions
human
I used to always stand when taking a piss, for about the first 20-25yrs of my life. I think i did this because it was imprinted into me that "only women sitt down to pee". 

Now I piss sitting down when at home or at my friends/family homes.
Primarily because Im a lazy bastard, and for two reasons.

First of all, its alot more relaxing to sitt, and i can check my Phone or whatever while draining the tank. And i have a pretty big bladder, so if i've held it for a while, its gonna take a while to empty on freeflow, and i cba to push... 
Second, im rather tall, so peeing standing up gives the stream too much height to gain speed, and No matter where in the bowl or water i aim, there is some backsplash of micro drops. And its alot easier to sitt down and rarely have to wash the floor, than it is to stand and Then have to wash the floor around the toilet alot more often, as these mini/micro drops quickly add up on the floor around the toilet to the point it gets visible.

When out and about in public, i piss standing up. Id rather use the urinal than sitt down on some random pub toilet.
gpt
There's a whole trajectory physics going around peeing. And yes micro drops on jeans doesn't look good.