Three effects I can see that haven't been mentioned yet:
Prostitutes have a significantly lower life expectancy than non-prostitutes - a mortality rate 40 times as high. Overall mortality rate for, say, a 25 year old woman, is about 1 in 2000. For prostitutes, it's thus about 1 in 50. Compound that for, say, ten years, and you get only 82% of prostitutes surviving past that point.
The lowest-class prostitutes would service the most men and have the highest mortality rate. If they have a 10% mortality rate per year (only five times worse than the average across all prostitutes) over a career of 10 years, then the whole cohort of women older than that point will have about 65% of them dead. After observing the horror of this, we note that death also renders one unable to respond to surveys.
Second effect: prostitutes especially but anyone highly promiscuous in general may have a hard time remembering just how many partners they've had, and may be poor at estimating it, even if they're not trying to minimize it. Especially when they need to discount for repeat customers by a somewhat difficult to estimate margin... it's a genuinely tough problem when you get up into the professional levels, at least without bookkeeping, which will frequently not be kept.
Third effect: prostitutes and ex-prostitutes, being especially disadvantaged, may be harder to reach.
Today I came across an article in the Telegraph that states that an average man has 9 sexual partners in his lifetime, whilst a woman has only 4.
Let's assume for a moment that in fact all these men and women are heterosexuals. In that case, each partnership contains one man and one woman. So, in total, the number of partnerships that women enter into is exactly the same as the number of partnerships that men enter into. Given a few other pretty well-known facts - that men are roughly as numerous as women, and live roughly as long, we deduce that on average men enter into roughly as many sexual partnerships as women.
There is, of course, a potential flaw in this - we know not all partnerships are in fact heterosexual. We know that heterosexual partnerships must have as many female participants as male, but we know no such thing about homosexual ones. Perhaps homosexual men have many, many more partnerships than heterosexual men, or women of any inclination? Whilst there is some evidence in favour of this concept, I don't really think it's going to be a big enough effect to skew the entire ratio this much. In order to have a ration of 9 to 4, most male partnerships would have to be homosexual ones.
A better theory is that the data is not very good. Perhaps men tend to exaggerate the number of sexual partners they have, even in anonymous surveys. Perhaps women tend to emphasise their degree of virginity. Perhaps the distributions are different - perhaps for example, most women have fewer partners than most men, but some women have lots? And perhaps this sort of information is lost in the survey?
Probably the only safe conclusion we can draw from these figures is that journalists and statistics don't mix.