Hi Your partner may be having an abnormal sexual behaviour called as paraphilia, with intense sexual fantasies .sort of perverted sexualities. Inserting any oblects can be dangerous as it can cause internal injury followed by infection.It is not under your control hence there are reported cases of perforation in to abdominal cavity via vagina. Hence discuss with him about his problem and consult a psychologist/psychistrist or/sexologist. You both can use your fingers after cleaning properly for insertion and masterbation. Hope you are convinced. If not you can send more queries  in private consult option.best wishes