If you're not into BDSM or any kink your partner may have you should say something. Yes they both consented to it but that doesnt mean they have to do anything that makes them uncomfortable.
Mokyeon and Nanjo need to get their shit together. Sunhwa need to see other partners because Mokyeons and Nanjos action have put him in danger. All three need to start acting and communicate like adults.
If you're not into BDSM or any kink your partner may have you should say something. Yes they both consented to it but that doesnt mean they have to do anything that makes them uncomfortable.