Question

Usually, in discussions of Jewish law about the prohibition of theft, the focus is on a situation where there is clear ownership of property belonging to someone else, and the discussion revolves around the extent of waiver when there is an assessment that the owner does not object. Jewish law is lenient in many such cases.
But what is the law in the opposite case?
That is, a case where there is no clear ownership claim, and there is also no clear legal-halakhic obligation, for example: giving a check at a wedding, where there is no absolute obligation, and it is not possible to sue over it in court.
However, in practice, the host does care about it, and acted on the assumption that every guest would pay at least the cost of the meal, say 400 shekels, in keeping with the prevailing social code, which is considered standard and taken for granted among the public at large. 
In your opinion, in such a situation—where there is no ownership or obligation, but there is clear objection and a widespread social convention—could someone who refrains from paying be defined as guilty of theft? If not, what would happen in a situation where, in addition, the host had attended my wedding and paid the 400 shekels—would I be able to demand that he pay at mine as well? And if not a legal claim, would I be violating some prohibition by not paying (beyond just boorish behavior)?