I am quite liberal but not stupid. My mother had a great defence for this, my 4 year old brother and 6 year old sister. She would send them in to my room every ten minutes. What she did not realize was I was far too scared to try anything whilst she was in the house.
That is not when you need to worry. They could behave whilst you are there and go out and have sex in the closest woods or field.
I would take a two pronged approach. Go in heavy with the consequences of what will happen if they disrespect your house with underage sex, but also give them a condom. There will be others who say this is contradictory messages, but I do not think so. I think it is portraying a message of " I am not happy about underage sex, but it would be worse if a baby got caught up in this". I would end it with a " by the way, if I catch you, you will be allowed no friends in my house what so ever!".
Teenagers will be teenagers, but that doesn't mean adults have to stop being adults. I had so much respect for my mother, I could not even have sex when my husband and I were house sitting and looking after my younger siblings for a weekend. It just felt wrong and weird.