A quick way to abruptly cut a conversation short, "ttyl" is along the lines of the police command, "Maintain radio silence," and it's loosely translated as, "My spouse/family/co-workers/friends are here. Do not send me anything else."
It is unspoken but understood that the receiver will simply stop and wait for something else from the sender.
In the conversation below, dessert refers to casual sex after dinner. Whipped cream optional.
Her: Hey, babe, he's gone tomorrow AM for 2-3 days. Kids at grandparents. Dinner & dessert?
Him: I'm in. Wherever you want. Ideas?
Her: Been to that new French place on main? Heard it's yum and very romantic. He's coming up driveway.
Him: Perfect. Dessert at my place.
Her: God, yes, please, I SO need dessert.
Him: Honey, I'm ready for dessert too. I want to eat as much as I can.