Its completely bizarre that a woman would react that way, even a woman that desperately wanted a grandchild... suggesting he marry a "whole" woman, is 1. an expression I don't think I've ever heard in my life to describe a female that can breed. He knew your situation and the odds of you conceiving prior to marriage didn't he? If he didn't, and you didn't know either... then thats neither here nor there. Many couples try and don't succeed at having children the natural way. There are many other ways to have a baby in this day and age and unless his mother lives under a rock she's sure to be aware of them.
Assuming you didn't misunderstand the conversation or context... (it can happen), I'd not fret over how she feels about his marriage to you. She's the one that will have to learn to accept that he loves you or lose him, he loves you, he's not going to leave you over mommy, not going to happen. You are the woman of his life now, she raised him, her job is done. If she wants to be an active part of his life, she will eventually have to come around to treating you decently.