For a girl to go through that sort of “abuse”—wow, she must be at the end of her rope (as in, exhausted all options to surviving). For the other partner to subject her to mutilation like that underlies a fairly cold and calculating person devoid of a soul. Yeah, this is a totally toxic relationship.
For the bitten girl to lie about her injury is a signal of victory for the biter... this is a sad story to me. The bitten person is setting herself up for future abuse because she’s establishing a pattern of hiding her abuse from others and accepting her fate in an abusive relationship.
The manga is well drawn, but the outcome isn’t a pleasant one.