It was an example. What I was trying to get at is that it is not necessarily an 'equal exchange' of consideration. I have no real knowledge of why these particular vegetarians are so, but one possibility is that they see a moral objection to eating meat. If so, for the meat eater to have a vegetarian meal when they visit has no moral cost to him, but that would not be the case when he visits them and expected meat. so it is not an equal exchange of consideration.