Generally when all your options disappear it's because your needs such as hunger or energy are too low. You need at least a minimum number of energy, food, hygiene, and fun in order to muster the will to socially interact with others.
As for the option for sex disappearing. If you are referring to the sex talk option that's not meant for imitating sex. That's just the trigger for raunchy conversation / jokes. Like all other talk options it can only be used once per day.
Once you unlock casual sex from enough dates. You should get a new interaction option with their girl separate from the talk menu.
