The inner-thigh biting schtick felt like shallow erotica writing at first, but there can be a slightly clever justification for. I hope Takano was thinking what I'm thinking.
The vampires here are big on secrecy and it's probably easier to hide fang marks on the inner thigh than on the neck. I am not very familiar with anatomy, but AFAIK the femoral artery, one of the largest in the human body, runs pretty close to the skin at the inner thigh, making it a good target.