Slytherins are known to be traditional. But as is the norm, there are always exceptions to the rule and sometimes those exceptions reinforce the norm.
- Narcissa grew up and saw what her fathers cheating did to her mother. And she swore that whomever she married would know better than to cheat on her.
- During her years at Hogwarts she had to make Andromeda stand down when her first boyfriend and the one after that cheated on her.
- The first one she hexed, nothing serious mind you. Just a little something to make him violently ill to the touch of a female.
- The second one didn’t realise she didn’t take kindly to being cheated on. So this one she hexed the same as the other and enforced a Slytherin Girl wide rejection. Some of the Ravenclaws took it up as well. These girls would not talk to him and would go out of their way to be snide and rude to him. This ban lasted nearly a year before he came back apologising and begging Narcissa to stop.
- This in turn made the rest of the girls be less forgiving on cheating partners, starting the Slytherin Sisterhood that dealt with these sorts of situations.