Let me put it in terms you might understand from your personal experience.
If you kissed your gay HIV+ boyfriend, even though there might be detectable virus in his saliva, you probably aren't getting sick unless you both have oral lesions, probably not even then, though.
But when you go out to your favorite gay bathhouse and get rawdogged by 20 dudes who also mix powdered glass into the lube, you will start out with a heavier viral load and are therefore much more likely to end up in the full-blown AIDS category more quickly.