Biting

FL bites the SML's jaw to shock him so she could push him away as the ice was breaking beneath them.

The ML saw the bite marks and bit the FL's neck out of possessiveness / jealousy and as a punishment / warning.

Ep 2 / 3

The full moon causes the ML severe pain. He bites the FL's lips because only a taoist's blood can relieve the pain, which is... a new one.

Ep 12

While tormented by dark energy, the FL bites her own hand. When the ML can't get her to stop, he offers his own hand for her to bite instead. After a moment of hesitation, she does bit his hand. Hard.

Ep 9 both

Ep 47 of 56 (20 of 29). ML and FL bite each other to create a symbolic bond.

ML bites FL's ear while placing a curse on her to have nowhere to stay except by his side. 

Spoiler: rather than a curse, it's a sign of devotion that links their senses so the ML knows when the FL is in danger.

FL bites ML's shoulder in anger. Episode 23.

Also lots of biting during actual fights.

ML bites ML's ear while drunk.

FL bites the ML's ear while drunk.

The FL links her life to the ML's so he cannot kill her. When one of them is wounded, the other feels it. This leads to some finger biting. 

Episode 3

FL bites ML's hand that he uses to cover her mouth to keep her silent.

FL bites ML's lip hard while drunk. Says she's eating strawberries.

ML bites FL's ear while pretending that he's going to force himself on her to scare her.

Ep 8

One ML ties the other up and they fight

The FL bites the ML's arm hard. 

Second couple  - SFL bites SML for revenge

One male character bites the FL several times to drink her blood. 

The FL bites another male character's hand in episode 14. 

Ring bites

FL bites the ML's hand. 

Ep 11.