First, take time to collect your thoughts. Calm down if at all possible before discussing the matter. Lashing out at your teen can only escalate the conflict between you.
Second, let your teen know how you feel in a caring way, and then listen to his or her feelings as well. You’ll get through to him better with dialogue than with a ranting monologue.
Third, realize the limits of your power and set limits as you are able. Teens don’t usually stop having sex just because a parent demands it. But you can let your teen know that you disapprove and will not permit his sexual activity in your own home.
There is a fine line to be sure between accepting the reality of your teen’s sexual activity and condoning it. But you can let your teen know that you disagree with his sexual choice, while letting him know, too, you care enough to emphasize the importance of safe sex and consistent use of birth control if he or she doesn’t choose to abstain.