It's at least as feasible for any given woman to be stronger than any given man as it is for said man or woman to have an 18 Strength in the first place. Your DM isn't just casually sexist, but he's got no understanding of statistics whatsoever.
Actually, wouldn't gender based strength differences largely just affect baseline strength, but not peak? Men find it easier to acquire muscle mass, but the maximum amount of muscle that can actually be packed onto a human frame isn't all that different between genders, given similar heights and weights.
This is basically it. Women are generally shorter, lighter, and not quite as strong as men, but there's no physical or biological necessity for the tallest, largest, and strongest person in the world to be male. (Actually, I believe the tallest person in the world is, in fact, female.)
That said, even if we were going to talk about a medieval world that didn't simply perceive women as weak because reasons, women are going to be pushed towards certain roles while men are pushed towards others because the medieval adventuring life has a nasty habit of amplifying otherwise small differences. Someone who is 2 inches taller and 20 lbs. heavier than someone else has a distinct advantage in a fight. Not much of one, but we're talking about
combat here, and when you're betting your life on something, you're going to want as many points in your favor as is possible.