Say two people (A and B) have been seeing each other for a couple of months.

Despite things seeming fairly serious, person A often states that it's not a relationship, that they aren't exclusive and hints that they both might meet other people.

If person B then sleeps with someone and tells person A. Does person A have a right to be annoyed or is it unfair to expect Person B to act like they are in a relationship despite being told many times that they are not?

Discuss...