How does/should exclusivity come about in (monogamous) relationships?

When entering into a new relationship, can you have implied exclusivity, or must it always be explicitly discussed? At what stage is it appropriate to initiate this conversation? In heterosexual relationships, do you think this responsibility falls to the man, the woman, both, or neither?

If an exclusivity agreement has not been made, and one party engages physically or emotionally with a third party, should this be discussed? Why or why not?

As always, anecdotes are encouraged!