When you get a woman willing to take cock in her anus, you need to fuck her that way regularly with a view to making that her standard form of sex. It gets easier each time and before you know it, that becomes her reality.
The first time I assfucked my chubby mature Filipina, she said it hurt when she shit for a week. She got more at ease with it as time went on. Better lubed and then ass-licking changed her mind. She even started giving me blowjobs whenever I came over.