Interesting point, thanks.
Two points: First, depending on how a hypothesis is framed, it can take contradictory data point to invalidate it. The example would be a hypothesis that "all sexism is necessarily misogynistic." If there is one case where this is untrue, the hypothesis is invalid. If a hypothesis is expressed more generally, such as "in many cases sexism is misogynistic" a single data point would not invalidate it (a hypothesis, by the way, that I would be inclined to believe). So taking an absolutist stand leave a position vulnerable.
Second, even the other half of this example does not necessarily mean that the father harbors contempt for a gender. Let's say the father does not sign up the son because he believes (mistakenly and with sexism) that all boys do not like dance, and therefore his son would be unhappy in the dance class. I'll submit that this too would be benevolent sexism, since there hatred/ contempt is neither expressed or believed. The belief could be completely misguided but not involve a general or specific hatred towards a gender.
To sum up, a person can be misguided and flat out wrong regarding sexism without having contempt in their hearts. I think we'll continue to disagree here, but I'll thank you for an interesting and respectful disagreement.
[And, by the way, my son takes dance lessons (hip hop) and loves it! : - ) ]