I had gotten my child sim through his teen years successfully, aged him up to young adult, and grabbed his boyfriend to add to my household as well. The boyfriend, Austin, had all the traits that made him good BF material - flirty, great kisser - and some he shared with others in the household: brave, artistic, virtuoso...
Well, all the things that Jaime liked in Austin were attractive to other Sims, too, once he was a young adult.
So, naturally, with a lot of Flirty in the household, people flirt. Never mind that Jaime's dads were hitting on his fiancé!
It never got more inappropriate than "compliment appearance" or "flirtatious joke", but I had to keep an eye on them.
Eventually, I took Petyr, the werewolf dad, and used "Edit Sim" to change his traits around a bit. He was going through a mid-life crisis (still is, in fact) so I took the opportunity to back off on his flirtatiousness and made him Charismatic and Ambitious instead of Flirty and Irresistable.
Crisis averted!
Both my couples Petyr and Tamlin, and Austin and Jaime, have conducted themselves well enough to have the "Eternally Faithful" moodlet and reputation. But when the butler started stirring things up again, Maggie the (grand-) mother (-in-law) wasn't having anything to do with it so I let her fire him.
