It is sexist if it is a must. I don't like gender roles, I view them as harmful to both genders. It's ok to chose to have an arrangement, I don't see stay at home moms or dads as inferior or oppressed if that's what they want to do. But I don't think everyone should be boxed into a specific category, even if there was evidence that in general gender X is so and so, there's always people who fall outside of the norm.
I don't fit my gender's supposed role: I have no desire of having children or to stay at home. So how am I supposed to go through life if there's a gender role enforced? My happiness and mental health don't matter?