You're over-analyzing this.
It's really quite simple and straightforward.
The science and research has shown that the HIV virus needs a set of near PERFECT circumstances for transmission to occur. If ANY of those factors are not present, or are impaired in any way...transmission cannot and will not occur.
The factors NEEDED for transmission (not some, ALL of them must be present):
1. For one, the partner you're having a sexual encounter with MUST be HIV+. The facts are, the vast majority of people do NOT have HIV. It's actually very rare, despite what people are taught. There are 1.1 million people in the US who are HIV +. The total US population is 316.13 million (2013). You do the math. NOT very likely that you're going to come across an infected person too often, if ever at all.
The people who typically end up infected are engaging in continued, frequent very high risk behaviors (IV drug use with sharing of equipment, males having unprotected anal sex with other males). Run of the mill sexual encounters of a heterosexual nature are very low on the risk list. Pretty much, if a person commits to using a condom each and every time they have vaginal or anal sex, HIV will NEVER be a concern. It really IS that simple!!
2. The virus MUST still be active (aka "alive") to allow for transmission. The virus very rapidly loses its ability to infect when it's exposed to environmental elements (air, pH, etc).
Everyone wants to know "HOW LONG???". The answer varies depending on the environmental factors, but the important thing to remember is, once that virus leaves the host, it begins to essentially "break down" immediately. The outer shell of the virus becomes damaged, which is the part of the virus that must be viable to allow for transmission. It's the outer shell of the virus that latches on to the T-cells in the receptive partner when transmitting the virus.
So, for argument's sake, even if the virus isn't completely 100% inactive, it's still loses its ability to infect once it has been exposed to the elements. This is why ONLY insertive vaginal or anal sex would allow for transmission, and why all other sex acts are not a concern.
3. There MUST be a portal of entry that allows the virus to get into the receptive partner, and most importantly, find its way to the bloodstream. That has to happen. There are only a couple ways to achieve this.
One is directly introduced to the bloodstream, like in an IV drug sharing scenario. The one drug user pulls BACK on the syringe plunger, to ensure he/she is in a vein, which causes blood to go into the barrel of the syringe. The NEXT user essentially injects that blood (which is well preserved in that barrel) DIRECTLY into their bloodstream. Many people worry about blood exposures. A "blood to blood" scenario is super rare, and it would have to be very significant to cause a concern...meaning no concern at the barber shop, or after getting a paper cut, etc.
The other is introduction to the body via a mucous membranes. Now, there are a lot of mucous membranes in the body, but not all are ideal for the virus to find its way in. The main mucous membranes involved in transmission are the urethral lining of a male's penis, the woman's vagina, and the rectum. There are other mucous membranes, like in the mouth, and nose...BUT, not all mucous membranes are created equal. You can read more in my health page about HIV and oral sex:
http://www.medhelp.org/health_pages/HIV/HIV-AND-ORAL-SEX-THE-FACTS/show/1278?cid=68
4. Finally, there MUST be a sufficient amount of infectious fluids to cause a risk. The ONLY bodily fluids that contain HIV are blood, semen, pre-seminal fluid, rectal secretions, vaginal secretions, and breast milk. There has to be exposure to a large amount of said fluids to even cause a risk....a drop or two of a fluid isn't a concern.
Hopefully this helped to calm your fears. I'd strongly advise not googling for information...that's only going to fuel your anxiety.
Again.....VERY simple, and not much to have to think about.....don't share IV drugs, and always use a condom for vaginal or anal sex (unless you're monogamous with a partner and are tested together routinely).