If you are a woman and concerned about your wellbeing while WWOOFing, then we suggest the following:
Choose a Host that is shown in the Host entry as 'Verified', as this denotes that the Host has to provided the WWOOF Japan office with a form of documentation that shows that they are the person they have presented themselves to be in their Host entry.
Communicate back and forth with a given Host that you are thinking you might like to go to, and through that dialogue, ask questions, raise issues important to you, check and double check, and ascertain for yourself if you can feel confident that the Host is the right place for you to go to.
Choose a Host that is a family with female members, rather that a Host that is a man living alone.
Choose a Host that is not in a remote place, but near other Hosts, or near a town, or a Host who has nearby neigbbours.
Always be careful, and if in doubt about a Host, then don't go to that Host, and find another Host instead.
If you ever feel in danger, call the police, without delay. The telephone number for the police nationwide in Japan is 110.
Report all undesirable circumstances to the WWOOF Japan office.
WWOOFing is not for everyone, and some people are better suited to go traveling on an organized tour of some sort, with tour guides, supervision, staying at registered hotels, etc.  If you are such a person, then you might not want to go WWOOFing at all.  The onus is on you to decide what is best for you.