At what point does the relationship become serious?

There is a man and a woman. They go for the pen and have sex, kiss ...
They spend the night with each other sometimes, and maybe not (even no one has asked anything from anyone).
But then a woman and says - how, are you, with another woman asleep? You are a goat, a beast and a traitor!
Although everything is as simple as a divine day, no one promised anything to anyone, even if they confessed their love to each other - then everyone has their own form of love, and not always it assumes bed fidelity.

So, the attention, the question: - At what point and at what stage of the development of relations, it's time to stop sleeping with others, or at least start to hide it and behave as if nothing of the sort (take calls without explanation, notify where you are and t .d.)?
Well, suppose, if everyone lives as if in their own housing - probably, the delivery of keys?
And if everyone is with their parents?
Or you must definitely say this point: "Now we are serious, remember this moment"?
Or are there more beautiful maneuvers?