I DM for a few different groups in a simultaneous fashion.
DM'ing is a breeze for most of those groups. We always have a blast, we have a lot of fun, and we always end our day looking forward to the next session.
For one group in specific, however, the situation is quite different.
A few weeks ago, I began to DM for a group of three people. I don't know them very well in any manner - we met over a boardgame/RPG meeting on my town, and set up to play Pathfinder 1e.
I'm usually pretty chill about most themes. I can run social adventures, combat fests, investigative dramas, romance stories, you name it. Most of the themes are fine by me, and I'm usually happy to adapt to whatever my players want to play.
Now, note that I said "most".
This group is trying to push content in the game in a manner that is making me really uncomfortable. While I usually don't mind to let any content show up in the game if everyone is okay with it, this subject is specially sensitive for me and the group insists in tackling it even after I said I wasn't having fun with the topic at hand.
I decided to leave the game.
However, ending a face-to-face game like this is somewhat new to me. I was never put in the position where I am the person that doesn't fit in the group, and I'm having serious issues trying to express myself regarding my wishes to leave their group.
So, I ask the titular question: How I politely say to my players that I don't want to DM to them anymore?