Yvaine is correct,
I was, like several other comments, sceptical about the 'everyone else' - it left me wondering whether the LW complaint would have been the same if the non-helping guest were, say, a daughter's boyfriend, or an older person's husband or boyfriend. LW's son is a family member, not a guest. I think in this kind of situation there is often a lot of 'unconscious' sexism, where women are often judged much more harshly than men for the same behaviours.
In the scenario I experienced, I would not have minded at all pitching helping to clear the table etc - it was a meal in coworker's home. My objection was to the double standards which were applied to me and female colleague, compared to the male guests. I think it is fine to accept offers of help from guests, I don't think that having first and second class guests is OK