Yeh, it's important to have the talk in the very beginning.
I would say right after the first French kiss. After that the interest is certainly there so both partners should be in agreement where to take it from there on onwards.
The fact that your partner did this to you shows that he has a very dishonest and sneaky side. He was being opportunistic. Even though you haven't had the talk, he knew that you were after exclusivity since you mentioned "No flings". He did this dating site thing with full knowledge of your point of view.
Be careful.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~