If you make your framework clear from the beginning, then you won't get hurt in the long run.
They may or may not be your only partner and can include sex.
The protocols and practices of dating, and the terms used to describe it, vary considerably from country to country and over time.
While the term has several meanings, the most frequent usage refers to two or more people exploring whether they are romantically or sexually compatible by participating in dates with the other.
What is the difference between "Seeing someone" and "Dating someone"? Logically speaking, a relationship should progress from being open and inclusive of all possibilities to one of exclusive nature with one person.
I think if you are searching for exclusivity, then you should ask for it (or rather, ask for where you stand in the relationship in regards to this matter).