can it be happening that the host have a strong will and the roper cannot controll her? in many cases host and parasite are share the body with coexist and if the host is sleeping or the parasite want sex they change their mind. even when the host have sex with a man her own will the parasite get semen so he musn´t take over. when a child is born are a parasite in this case? i´am not sure with this but in that case many mutants are born.
i think a lot of monstergirls are bisexual with human womans or sometimes with other monstergirls. a man can have a human woman and monstegirl as wife at his side in that case sex have more ways. if the man cannot anymore the monstergirl have sex with the woman to still their lust or both of them teach the monstergirls lesson of sex. if the man is not at home the monstergirl and woman have sex together if they want. well there are many other ways possilble. maybe some monstergirls love human woman more as man and married her if they want. love between a human woman and monstergirls are not unusually. or if a monstergirl not found a man for sex and a woman cross her way she catch her and sex with her until she has enough.