Learn English – What would you call someone who imposes on other people’s generosity

For a word that can refer to both married people or people in a relationship, you can use cheat, cheater, two-time (informal). Two-time is also used as verb, as reported from the NOAD: He was two-timing a fiancé back in England.

For people who are married, you can use adulterer (which is gender-neutral) or adulteress (when referring to her).

The phenomenon you speak of has a distinct studied name which I remember from a psych class from long ago - ad hoc hypothesizing. If I remember correctly it is when you compensate for abnormalities or issues by creating false rationalization(s) about a topic - in basic terms, changing your reaction based on outcome. I would just call them a liar. These people are certainly pathological.

The thing about ad hoc hypothesis is that they are stated in a way that is almost impossible to prove wrong. In your example Mary states she purposely left the food in her teeth. It is almost impossible to discredit this.

Now if the person is just doing it on purpose to be funny they would be a smart-ass.

Also for a person who is kind of in-between the liar and smart-ass, the person who change an argument based on the end result would have circular logic. In your specific example this could work, but we know that 99% of people don't leave meat in their teeth on purpose.