The man and the woman should establish exclusivity and commitment mutually. Oftentimes, an effective way to go about this is to set the expectations from early on in the interaction.
In the early stages of dating, you should talk to one another and make sure that you are both looking for the same kind of relationship.
As the relationship progresses, start doing the kind of activities that two people in an exclusive relationship would do.
Meet each other’s friends, family, coworkers, work out together, attend weddings or birthday parties together, travel together, etc.
Consistently engaging in this kind of behavior will ultimately lead to a mutual understanding that the both of you are in an exclusive relationship, even though the both of you are yet to have that particular conversation.
People will eventually start asking the both of you if you are in a relationship and if the feeling is mutual, you will find that the both of you will acknowledge that.
This would then be your opportunity to have a brief discussion after the fact to summarily confirm that you are both in an exclusive relationship.