I do not think that when man and woman are having illegal relation ship they will not have aids even if all precautions are taken.
The best drivers will have car accidents even if they very careful and smart.
In our belief God gives us his Mercy that any legal relation between healthy man and health woman according to our belief there will never be aids, Why?
Because Gods wants always the best thing for the mankind.
In the holy Quran God says if he wants he will bring other people than us, but thats not what God wants here is holy Quran verse:
Ibrahim

Hast thou not seen that Allah hath created the heavens and the earth with truth? If He will, He can remove you and bring (in) some new creation; (19) And that is no great matter for Allah. (20) They all come forth unto their Lord. Then those who were despised say unto those who were scornful: We were unto you a following, can ye then avert from us aught of Allah's doom? They say: Had Allah guided us, we should have guided you. Whether we rage or patiently endure is (now) all one for us; we have no place of refuge. (21) And Satan saith, when the matter hath been decided: Lo! Allah promised you a promise of truth; and I promised you, then failed you. And I had no power over you save that I called unto you and ye obeyed me. So blame me not, but blame yourselves. I cannot help you, nor can ye help me, Lo! I disbelieved in that which ye before ascribed to me. Lo! for wrong-doers is a painful doom.
here is the reference:
http://www.quranexplorer.com/Quran/Default.aspx