At 16 I had a boyfriend who used to stay over.. we even went away for a week together. We used to talk, watch movies etc. We were together for 5 months and we never had s.x. I actually think most smart teenagers wouldn't have s.x in their parents house. It'd be awkward.
If kids want to take the plunge they're going to do it unless you chain them to a chair inside your house. All you can do is explain safe s.x and tell them it should be for when they're older and more mature.