Are you joking dude?
HIV can be transmitted through ANY exchange of body fluids.
"really not going to happen" is not even an opinion.
A virus is tiny enough to pass through any cut in the mouth, even an abrasion left from brushing the teeth. In most people, there are abrasions in the mouth, on the inside of the cheeks, from rubbing against teeth. Most people also have tiny abrasions on the edges of the tongue. The inside of the mouth makes up for its wet condition, and exposure to hard edges (teeth) by being a prolific producer of saliva, mucus and surface skin cells. This is why the mouth is always "shedding" old cells, and can easily be swabbed for samples when DNA samples are required, etc.
While the chances are low, still better be safe about it.
Chances are low that I will get run over by a Greyhound bus if I run out in the street without looking. That's not a good reason to go running around in the street.
If one's sex practices are unsafe, they are going to put them at risk anyway.
Besides all this, chances being "low" has more to do with pure statistics, anyway. It's an established statistical fact that certain practices have been linked to contracting HIV, but those statistics are based on how many people have reported their means of contracting the virus. Many more people contract the virus through other means (i.e., heterosexual or homosexual intercourse through vaginal or anal penetration), simply because those other means are practiced more frequently, as a percentage of cases, with more fluid being exchanged.