You might try reading the OP, which specifically mentions the 18th century as a focal point. What gives you the idea that prostitutes mostly engaged in oral sex in the 1700s? Please provide some cites.
Certainly literary accounts, such as Fanny Hill and Moll Flanders, indicate vaginal intercourse was common and pregnancy a concern.
Indeed, it wasn’t completely unheard of for a beloved royal mistress to be enobled or her bastard son be made a noble.
Dudes- the Romans had hookers that specialized in Fellatio.
And for modern times, I admit I do not know much about them first hand, but the HBO specials about streetwalkers have made it clear that fellatio is the most common type of sex paid for.
This study also mentions prostitutes using fellatio or anal sex to avoid pregancy:
http://www.springerlink.com/content/u85530q62m143514/
So what? Only a minority of prostitutes (it looks to be maybe 10% or so) are listed as offering fellatio. This implies that most did not make it a specialty. If fellatio was in fact offered as something special, this would tend to prove the point that most prostitutes in fact engaged in vaginal intercourse.
In the US in the 21st Century. And of course streetwalkers are more likely to offer a quick and simple act like fellatio that can easily be performed in an alley or the back seat of a car without disrobing. Those working out of brothels can more easily offer regular intercourse.
From everything I have read on the subject, in most times and in most places vaginal intercourse is the form of sex most regularly offered by prostitutes. Other forms of sex may also be offered as a specialty or in special circumstances, as in the case of streetwalkers.
The abstract, which is all your link shows, doesn’t mention fellatio or anal sex at all. Even if some prostitutes do use such practices, that says nothing about their frequency relative to vaginal intercourse. Since most Colombian prostitutes have easy access to condoms, the majority surely don’t need to rely on such methods. (Most prostitutes here in Panama are Colombian. There has been a recent series of articles in local newspapers here on the trade, which is semi-legal. According to the articles, most prostitutes engage in vaginal intercourse and insist on condom use, though they may offer other services as well.)
Well, do you have a cite that shows that most hooker sex is vaginal? 
Sorry, I guess you don’t subscribe. Apologies.
Anyway, let’s not get into one of our classic hijacks. 
All that is needed to say is that oral or anal sex could be and was used for birth control, and so were condoms.
Prostitutes used contraceptive sponges.
I could get access to it, but most people here can’t since it does require a subscription. The link you posted didn’t support your statement.
This is directly related to the question in the OP, so it can hardly be considered a hijack.
If that’s all you had said, I wouldn’t disagree. But your claim was that hookers do mostly oral sex, apparently based on what streetwalkers do in the US today, which isn’t really relevant to the OP.
They had pennyroyal, too. No way of knowing how its value became discovered, of course.
BARTENDER: Say, do you mind if I ask you a question?
HOOKER: Shoot.
BARTENDER: Do prostitutes ever get pregnant?
HOOKER: Sure! Where do you think bartenders come from?
My impression was that in the 18th C. the incidence of syphillis among street-walkers was so high that the life expectancy of prostitutes was fairly low. Furthermore, syphlitic infection along with high rates of alcoholism would certainly combine to keep both miscarriage rates and infant mortality rates very high, I would think. Add this to the fact that SURELY these women were suffering from reoccuring bouts of untreated PID, which would drive down conception rates and can lead to infertility, and I suspect that for many 18th C. streetwalkers, pregnancy was the exception, not the rule.
I’m trying to remember the cite, but I’ve read articles that back up DrDeth, at least in the Wild West. I remember the fact that most cowboys asked for blowjobs instead of vaginal sex from the hookers in Dodge City and other frontier towns. Now where did I read that…?
Probably something by Larry McMurtry
I heard that the Romans used half an orange as a form of Dutch Cap
Ergh, depends on the place. In some countries, BJs are extra.
(What’s greek, btw? I don’t want to google it at work. In Spanish a French is a BJ…)
Greek = anal.
French = oral.
English = whipping/paddling.
German = In Poland.
…
I saw an episode of Wild West Tech last week that talked about brothels and hookers in the old west.
They mentioned condoms (and showed some, not exactly what we are familiar with) and showed an item that was used as a primitive diaphragm. I was about 1/2 asleep at this point, I think it might have been a shell, but I don’t recall.
Has anyone mentioned Pessaries yet? I seem to recall seeing something about Margaret Sanger getting into some trouble for recommending or dispensing pessaries for contraception.
A number of streetwalkers in 19th century London would try to get away with having the man press between their thighs but not actually penetrate the vagina. If the legs were together tight enough (or perhaps more if the man were drunk or inexperienced enough) he wouldn’t know the difference.
I’m not sure how practical it was, but many of them used handkerchiefs as a kind of barrier. Certainly not as effective as condoms, but then the lower class women probably couldn’t afford them.
Abortions, as already mentioned, were common place. One common dodge in the law for those selling abortifacients was to advertise a product as some sort of special tonic for some other purpose but to give a very helpful warning that, as great as the product was, under no circumstances should a woman who is pregnant use it, as it had been shown to terminate pregnancies, and of course that would be a shame, so ladies be sure now…
“*Fine *young boy?- well he should be fine, as he was strained through a silk hankerchief!” 