People from America travel to other countries when they have STD's. There isn't any proof that people from foreign countries would be less honest about disclosing their STD status to potential sexual partners. Also, it is not likely that a foreigner would be more compelled to rape anyone here than anyone else.
At first I thought I'd write in support of allowing it, but then I realized that letting people in this country with any kind of communicable disease is a potential threat to our national security in the form of germ warfare. While it is probably far-fetched to think that a hostile nation would try to infect us to death, it's not entirely outside the realm of possibility. Better to be safe than sorry and lock those foreigners out unless they can demonstrate they have a clean bill of health.