It’s stereotyped because it’s based on a discriminatory attitude that women are in some way inferior to men.
It doesn’t remotely suggest women are inferior; it merely suggests that the person wearing it, being female, might end up being married to someone who was very keen on football.
You can’t even argue that it assumes that women are less likely to appreciate football than men (which is a stereotype but not discrimination and in my view would make women superior anyway), because that would entirely depend on also assuming that the woman was married to a man. These days, that is far from certain.
The only meaning you can derive from the cap is that the woman in question is likely to marry a dedicated football fan. But feel free to feel aggrieved by it.