When certain parts of our bodies are touched, certain enzymes and chemicals that trigger sexual desire are released into our system. The more our bodies are stimulated, the more chemicals are released and the greater our sexual desire grows until it becomes a virtually unstoppable flood.
When a clandestine couple kisses, it is much more likely that sex will immediately follow. This is because there is much more risk. Once you have kissed secretly, it is very difficult not to have sex. It would be impolite not to have sex. You have to show that you are serious about each other.