My parents said no to sleepovers when I was 16. But I pointed out that if we wanted to have sex we could do it whenever not only by night... Perhaps, rather than forbidding him to sleep over at his girlfriend's you could tell him your concerns? Maybe he (like me at his age) has no plans to do more than just sleep?
I respect your values and understand your dilemma. You want to set boundaries based on your values. But your son is old enough to start to form his own values. You forbidding him to do something that he feels is ok could make him feel resentment. My advice is to keep the discussion alive. My experience is that it was easier to respect my parent's decision if I understood where they were coming from. Explain to him why you feel the way you do. Perhaps you have already done that and then it should be end of discussion.