This will sound awful, but about 20 years ago, we lived in a flat with a stair. It was basically fashioned the same way as a house.
Of course the toilet was upstairs. I enjoyed a six pack at weekends. Most of the time I would climb the stairs to go to the toilet, but there were times when I was watching a decent film and couldn't be bothered climbing the stairs, so I just slashed in the kitchen sink. The good lady was none the wiser lying up in her bed.
I only did that several times I might add and always ran the tap for hygiene purposes of course. It wasn't my routine if you are deeply worried.
I now live in a flat and the toilet is conveniently near thank goodness. This thread has had me in stiches. How low can you sink lol 😂😂😂