I would like to point out the pachyderm in the room that no one has mentioned, the one over in the corner wearing the hot pink plaid suit and tap dancing while playing the soprano saxophone. Kind of hard to ignore I would have thought. The BBBJ is the center piece of the Hobby because most Hobbyists are over forty and need a warm wet mouth to get and maintain a serviceable erection. Sometimes that is true even with chemical reinforcement from Viagra and its workalikes.
Lots of Sporting Gentlemen prefer a BBBJTC because their erection will soon go away inside a condom while they can passively lie back and be “serviced” to “completion”. Or, if the call of their “Selfish Genes”
http://en.wikipedia.org/wiki/The_Selfish_Gene
to go through the motions of penetration is loud enough there is the two step shuffle: BBBJ, CG, BBBJ, “Mish”, BBBJ, K9, etc as the BBBJ enables or restores the elusive erection and the condom takes it away. No rational person will have genital sex without a condom so the BBBJ is the closest thing to condom-free sex that is available to participants in commercial sex (and non-commercial, non-drunken sport f*cking).
There is also a potential element of perceived emotional closeness with someone willing to take the center of your sexual self into her mouth without insisting on an artificial barrier, however thin. It appears that for the youngest sex workers, doing CBJ-only is the updated version of what not kissing customers used to be, a dividing line between work and sex with those with whom a real relationship existed or might be hoped for.
Allowing the ejaculation she worked so hard to cause to take place intra-orally hugely reinforces this perception of intimacy, even if just for the moment when it occurs. Swallowing makes it stronger still, potentially for both the partners of the moment. Makes it more dangerous too but nobody really knows to what extent. That is because no researcher is going to run a study that involves pairing infected people with uninfected in various ways sexually to see what the relative transmission rates turn out to be. Ex post facto case studies are to best that can be done ethically and they indicate the risk associated with condom-less oral is low but not zero. Probably herpes is the greatest risk and nobody dies of that. There might be a residual risk of oral cancers associated with a few of the hundred or so HPV viruses. Incidence of throat cancer has been going down, the percentage of tumors examined by research studies that have HPV markers has been going up. Probably someone who does not smoke or drink alcohol but has lots of oral sex is safer than an individual who has no oral sex but does smoke and drink but no one can say for sure. The beauty part with respect to HPV is that the Gardasil vaccine, although intended to prevent cervical cancer probably protects against HPV-related throat cancer. Nobody really knows since it hasn’t been studied, partly because the incident of throat cancer is low.
Sophia is almost certainly right that the risk, whatever it is, weights mostly on the orally receptive partner. That is one of the several reasons we pay and they are paid.