I am very happy to see an informed and cogent discussion of this topic on this Board.  
 Qualifications (or lack of same): Two doctorates, one of which is a medical degree. Not an expert on human sexually transmitted diseases. Used to teach histology (structure of the tissues of the body) to medical and other health-care students.  
 Both internally and externally there are cellular barriers between the surface (outside world, which includes what is in the digestive tract) and the bloodstream, but routinely there are breaks in those barriers that our body is capable of readily repairing, and immunological defense mechanisms to cope with the momentary breaches. However, there are certain agents that, once there is access, can’t be eradicated by our defenses (see for example, the viruses referred to above). The risk of acquiring a virus into the blood stream is a function of the amount of virus and access to an open blood stream portal. Keep in mind that it only takes one virus particle (HIV or hepatitis viruses for example) to make it to a site where it can survive and proliferate to do irreparable harm to you and your hobby associates or loved one(s).  
 My perspective receiver (oral): Virtually every adult would have some gingivitis (http://en.wikipedia.org/wiki/Gingivitis), which could be a portal to the blood stream. If you brush your teeth (preferably a soft bristled brush) and floss (both of which we all should do daily at least) and see red in what you spit out (assumes not having recently eaten something of that color or have red in the toothpaste), you have capillaries leaking blood into the mouth and likely have inflamed oral tissue somewhere. Even if no red is seen, you likely have some red blood cells in your spittle after brushing, flossing and even eating foods which can damage the oral mucosa (e.g., piece of bone from a pork chop, sharp cartilaginous rib of a fish or hard material in popcorn). We pretty consistently experience some amount of oral abrasion that results in breaches of the protective lining of the mucosa. (I consider gingivitis to be a chronic localized abrasion.) So, to me it doesn’t matter that the stomach produces acid that can kill some viruses. We can be defenseless before that point.  
 My perspective giver (male): I assume we all recognize that most enjoyable acts involve some kind and level of abrasion of the male organ. The penis is pretty well designed to cope with that, and there is a layer of epithelial cells that is a barrier, but they can and are abraded. I bet most guys have occasionally experienced a sore red area after masturbation, especially if they have used inappropriate things for lubrication (DON’T use shampoo!). As you can probably also appreciate, the epithelium is not as thick on the penis as some other places on the body ((interesting side notes - scrotal skin (epidermis and dermis) is the thinnest, and the density of sensory nerves is not most dense at the head of the penis but on the surface of the cornea)). Damage to the protective barrier can and probably almost always does occur during sexual activity (premature ejaculators may have an evolutionary advantage here!), but we generally don’t notice it (however, who hasn’t been scraped by teeth?), and our bodies quickly repair it. However, all it takes is one virus particle to get access to your bloodstream and proliferate to do you in. So, if you have a breach, and she/he has virus in the mouth, vaginal secretions, tears (she/he is crying from the joy at having your amazing organ orally?) which hits that breached spot, gets access and reaches the promised land, you’re doomed. Yes, a number of things have to be right for the virus, but the more chances taken, the greater the chance of the planets being aligned for the virus.  
 If one applies alcohol (what concentration? diluted bleach would be better, but how many would sign up for that, and what would mister happy look like after doing that 100 times?? and, you need to allow time for recovery from alcohol or bleach treatment as both can damage the surface barrier) post-coitally, I would expect to hear an ouch! a number of times (due to surface breaches allowing access to sensitive nerve endings). Yes, it might work, but also might cause additional damage to a site that was borderline, that would allow viral access. (Reminds me of attending a party out in a field as a teenager (late 60's, rural america) where a couple was DRINKING Listerine. What's up with that I asked? Well, they had hot lovin relations and were sure they were warding off a potential venereal disease.)  
 Professional Expert Opinion: If you donated blood prior to engaging professional services, you know that there are questions about engaging in sexual activity with professional providers. If you have done so, they don’t want your blood. They don’t care if you only did it once or used protection. Professionals responsible for keeping the blood supply safe deem the risk too high.  
 My Personal Perspective: I’m in the Dallas/Fort Worth area and would love to experience the talents of some (no, ALL, though I do have a priority list memorized) of the legendary performers bareback, but won’t unless I get a terminal diagnosis which doesn’t impair my ability to appreciate the act. In that case it won’t matter to my life (though I would have to stop having unprotected sex with those I care about), though I would also be truthful and say to the potential provider (first recipient gets a free pass) that my organ has been ministered to by some of the most skilled in the art, so they can make their own assessment of the risk I might present to them.  
 So, for me, it is too much like Russian roulette and I haven’t done it. I would like to, but don’t want to take the chance due to the risks as I see them, for me and particularly those I care about who are kind enough to share their intimate selves with me. As has been well stated above, these are personal risk assessments and choices. Hopefully they will be informed ones.  
 (While writing this I was struck by the realization that I was listening to music composed by a genius, Beethoven, who lost his hearing and later his life, to a sexually transmitted disease.)  
 Safe sex and a long life and much love, where our needs get met without harm to others,