What you did was fine, I would have done the same thing, however, all you can do is talk to him/them about it, to abstain from it (if thats what you wish) although, in relatively if they want to do it, they will. So either discuss abstinence for a certain period of time, or, all together, or speak about safe sex and the consequences if this approach isn't taken. be blunt and harsh, let them know reality, its not a fantasy i.e porn but the real deal.