"This is inappropriate", says my Sim's father

@EeveeTinn Interesting points. I agree, it would be a fun and appropriate addition to have some nuanced levels of 'allowed and welcome' behavior within families and within relationship levels when it comes to food access, bathroom use, and other needs being met... and just 'making themselves at home. Even if this was done within a setting per sim, invitation each time they come to the lot by a household member, or something along that line. (Not referring to 'woohoo' overnight invites.)

It would also be nice if we could have a guest bed or bedroom that has a bed not assigned to the household that could work for overnight invited guests, friends' sleepovers for kids, etc.

@EeveeTinn  Yeah, this bugs me too.  I feel like there should be different rules for adult children who moved out last week, your child's fiance, etc. 

My first time running into this was also the most hilariously awful.  I had a teen romance, both parties aged up, and then I realized that the parents didn't know each other... while planning the wedding.  Her parents exist!  I can't not invite them!  So brilliant me decided to hold the wedding at the other parents' house.  Great, right?  Only I forgot a cake.  😉

No problem!  The other parents own a stove, and... "THIS IS INAPPROPRIATE PLEASE STOP!" 

Because I was a n00b, after the couple exchanged their vows I ignored the bride's father and had Mom keep making the wedding cake, so the bride's parents threw the entire bridal party out into a blizzard, and everyone raced around in a panic and almost froze to death before I ended the event (Game:  "Are you sure you want to end the event early?  Everyone is having such a great time!" while my Sims run in circles in a blind panic and turn blue) and we traveled home.  But not before Mom peed herself in the snow.

This wedding earned a silver. 😉  Also no one died, which was its own reward, I suppose.  I now count any non-fatality Sim wedding as a success.

@xochiquetzl_xkvn Oh my gosh! I loved your wedding story... and I also hate weddings and birthdays in the Sims and have avoided them like the plague and kept them completely private for years.

I had nothing but bad experiences if I tried to run a birthday party or wedding when I was new at the game! So... about a year and a half (?) or so ago (for a challenge) @PugLove888 and I decided to team up and do one. I made the venue lot and she made the household, which included both sets of parents and their children. I moved and played them in a duplex I made so they all knew each other well. Then I ran the wedding in the venue and it was actually a lot of fun (I'm sure a lot was because I controlled all of the main/key participants!), and everything went so well!! https://imgur.com/gallery/4mFHgX2

Even so, I still do elopements and only 'age up', as that was not a typical situation. 😉🥳

@SheriGR  I've since done the Party Animal aspiration, so I feel like I've mostly gotten the hang of it, but I still find Kava Parties (for Beach Life) to be, ironically, the most stressful.  LOL!  Once you get the Party Animal reward trait (Perfect Host) and/or a good reputation from the Get Famous pack, it's a lot easier.  It still won't let you bake a wedding cake at your new in-laws' house, though!  😉  (Always Welcome.  Yeah.) 

While hilariously awful, that wedding's not the worst social event I've thrown.  While I was doing Party Animal with my Super Sim, a guest decided to go swimming in the snow on New Year's Eve and died.  Nooooo problem! I thought, and had Mom Transportalate up to Grim with a death flower... but she decided to witness death instead of giving it to him (autonomy, sigh).  Because we're attached to Brent Hecking, we spent the rest of the party Necrocalling and Dedeathifying him and earned a bronze.  I made sure he was properly back in his own household and married to Brant again, and had him comfort Brant.  At two AM, they both had popups of "New Year's Eve was AWFUL!"  Sweeties, you said a mouthful. 😉  (They're still marked played in that save. And yes, I had EVERY life insurance possible on hand, lol.) 

@EeveeTinnYeah, I would really love to see the game acknowledge family ties outside the current household for more than who you can or cannot date.  Like, yeah, Grandma can cook in my kitchen if she feels like it, as can adult kids who moved out.  Also, cooking is inappropriate but other Sims come over and take mud baths in my tub and other things that are, IMHO, a lot more inappropriate than baking cakes.  I had one married couple consummate their marriage in grandma's bed! fortunately, grandma never found out, lol!

@EeveeTinn  Yeah, if Vlad or Caleb or Lilith knocks on my Sim's door, I know they're there to...

...use their computer.  LOL!

If you own a game pad Vlad will never bite you no matter what you do.  He just wants to play Blickblock for 12 hours and then leave.  😉

(I used MC Tuner to forbid visitors from brewing herbalism potions and leaving the jars all over my yard.)

I had the very same problem. I started dating a sim that I created and to try and to try and boost the relationship, I tried cooking with her. Turns out, cooking is too inappropriate. After that I accidentally started a fire (I blame the cat) and I went and took a shower and she kicked me out for being too inappropriate. Bruh. Game, remove this feature.

@OldyMoldy I'm not saying this is what's going on in your situation, since a number of things may queue that message (including broken CC/Mods), but I have had times that I got that notice that come to mind:

1. I had a household that purchased a restaurant that I made (that I also made bedrooms, etc on so they could maintain themselves there). They had a separate home, but I ran them from this restaurant lot [using the whole workaround thing you can do with purchased retail stores where you bring them to the retail lot and run them from there, but when they leave the lot (work, school) they return to the 'technical' home lot and you can click on their image and choose 'bring here' and they come to the store you're using for a home]. Well, this seemed to be working for the restaurant until the married couple started having one or the other refuse to get into bed with their spouse with that 'inappropriate' message. (I also had a Sim that refused to leave when the restaurant closed.) I stopped playing on that lot and don't have my Sims 'live'/stay on a restaurant lot now.
2. I have had more than once where I was playing a non-household sim (that was at the other Sim's home) that got that message for using the shower (and maybe the bathroom) and other functions in the household (I think maybe cooking, if they try to sleep/nap, etc.)? I think in this case the Sim I was playing was an adult child of the household Sims that had moved out, and I assumed they could do these things... but it seemed rather hit-and-miss as to whether something was appropriate for them to do in their parents' home after they moved out?

Anyway, I think I have had the message other times, but those are the two that come to mind. In my case I did not have Mods or CC. Not sure if that's any help, but it's a thought. Now if you have that Sim come to your lot you should have the option in the pie menu to have them cook with you (to boost the friendship).