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As a parent you need to separate out 2 factors in this: the very real need to teach your son to manage the risks he's taking on by becoming sexually active, and your natural discomfort in your baby getting all grown up and having sex (or not: you actually have no idea what they did or didn't do in his room).
The first issue is something a parent can and should be heavily involved in. Make darn sure that your son understands pregnancy, STDs, birth control, and so forth, and has resources on how to use them. Make sure that your son understands some of the emotional ramifications of what he's doing. And make sure that he's exercising a fair amount of discretion when it comes to the people he's having sexual relations with. You can set limits on when and where as well: the night before the SATs would be a bad idea, and definitely not in anyone's bed but his own, that sort of thing.
The second issue is an entirely different one, namely your immediate emotional gut reaction which says, in short, "Son having sex? Ewwww." Just like as a kid your gut reaction was "Dad having sex? Ewwww." And sorry, but at some point almost every parent has to accept that, if only in a purely intellectual sense.
The good news is that you have a year to help your son deal with this issue. When you think about it, in a year from now you won't have the same amount of influence over his decisions, so now is your chance to teach him responsible sexual behavior, before he goes off to wherever he's going after high school.