her conniving already stole a relic (waaaay bigger offense) there’s no need to throw in the “YEAH AND SHE SHARES MEN AND STIIIIIINKS” like bro, really?
Let's consider the setting of this manga first. This is a Victorian era with a mix of fantasy where human nobles already have fiances at a young age so it's taboo and utterly wrong for the little sister to share a bed with 3 different men who have fiances especially when one of those said men is literally the crown prince aka ex-fiance of our MC. You mentioned poly but poly relationships have people know each other and gave their consent if they want to share someone / or be in a poly relationship but in this case, the crown prince was shocked to find out then and there that the little sister bedded his own two aides so therefore, this is under 'cheating' or 'triple-timing'.
Honestly, based on every dragonoids in different mangas that I've read, they all have sensitive sense smell so if he can smell three people on the little sister then that means she recently or done multiple times in the sheets with these men for it to be overwhelmingly smelly and cause a great shock to this guy. So I guess this is karma for the little sister for touching someone who isn't hers to begin with.
why do we gotta slut-shame.. humans are freaky lil bipeds who fuck for FUN.. let alone the countless species who are promiscuous (most common breeding pattern). like BRO, poly relationships EXIST yet get shit on HEAVILY by most genres (or fetishized).
you would thing a dragon, who is often associated with nature and the “natural order” wouldn’t be such a PRUDE