You are right on the money in your answer. Early on I found myself saying the same thing: “I’m not a lesbian.’ I’m not attracted to women and likewise I don’t find a man dressing like a woman sexually appealing. I suppose it’s the easiest way (saying I’m not a lesbian) of expressing that I don’t like a man dressed as a woman.
If a wife starts dressing masculine and acting more masculine, I don’t think it would be unusual for the husband to say he isn’t gay.
It’s also a way of pushing back and expressing discomfort as it is not what you thought you signed up for.
I will say that the more time and patience you give your SO can make a world of difference. I now enjoy painting my husband’s nails and am so used to seeing him in panties that I do a double take when I see him in boxers.