I'm not sure I can follow you here. This looks like circular reasoning.

I'm not sure what RobBB meant, but something like this, perhaps:

Utilitarianism doesn't have fundamental concepts of "obligatory" or "supererogatory", only "more good" and "less good". A utilitarian saying "X is obligatory but Y is supererogatory" unpacks to "I'm going to be more annoyed/moralize more/cooperate less at you if you fail to do X than if you fail to do Y". A utilitarian can pick a strategy for which things to get annoyed/moralize/be uncooperative about according to which strategy maximizes utility.