I have two house rules based on my pet peeves.
1. No evil characters at my table. You can be good or neutral, but not evil. I had too many adventures derailed by constant note passing and backstabbing. I didn't spend several hours preparing for game day so I could read everyone's notes about how they are going after each other. Granted, this was when I was was much younger that I had these problems. Though the rule still stands.
I used to say this but I don't find alignment to be helpful in 5e. I don't disallow alignment as much as I ignore it. It is about as important on the character sheet as your eye color.
Instead I have some general table etiquette:
1. No sexual violence
It can be part of a backstory, but I have no interest or tolerance in PC's engaging in rape.
2. No detailed sex roleplay
If it ever comes up, my games are "fade to black" when it comes to romance. I'm not going to be roleplaying a sexual encounter with my players. Discussing with a 40-something-old how he touches what is not my idea of a fun time.
3. No harming children
Obviously, it is a dangerous world and children die and get hurt and are used to threaten parents. But I want my players to be the ones that put an end to it, not engage in it. As a father I get irrationally angry just reading about people who hurt kids--I'd be inclined to punch out a player who roleplayed harming kids for the LOLs.
4. Constant backstabbing and PvP
Players have their characters do things in their own interests from time to time, over the party's interest. They keep secrets from one another. They don't all always share treasure equally. But I'm not interested in a PvP game. If the players are spending their time just going after each other, I have other games I would rather play that make that style more enjoyable for me. I've never enjoyed playing in or DMing that kind of game in DnD.
Beyond that, I'm not particularly prudish in my games. But the above rule might seem capricious to some.