Right okay since this is my first /r/AskHistorians post let me give you my credentials. My degree is actually in nursing, but I have done extensive historical research in three main areas. 1) History of sex and Sexuality 2) History of HIV/AIDS and the medical implications and 3) the history of nursing particularly in Britain and then Canada. (I'm also a hockey history buff but haven't done enough actual research in order to call that a main history knowledge). Here is a recent, sourced, comment I left in /r/askscience about HIV transmission

This falls under my first category of knowledge.

So the very first thing you need to know about genitalia is that it's naturally self-cleaning. Vaginas have a natural slightly acidic PH (cause by lactobacilli) and create mucus that "flush" the vagina on a regular basis. Penises not only have pre-ejaculate that acts as a "wash" of the urethra prior to sex Urine also naturally cleans the urethra after sex. So the area is naturally cleaned by the body.

As for the exterior of the body then it's really dependant on the place and era in question. Most of my historical research is based from the 18th century onwards. However certain assumptions can be made based on known hygienic practices of each era. We also need to remember that cleanliness is relevant if you look at 18th century France, unwashed bodies were the norm so there wouldn't be anything unusual with having sexual relations with a person who hadn't completely washed in weeks/months/years. As a recent example, pornography from the 70's and 80's typically depicted women with pubic hair. In pornography produced today, most women have no pubic hair, or if they do it's usually trimmed and carefully shaped. If you ask most of the younger population they will tell you that pubic hair is "Gross", sexual trends and norms can and do change rapidly.

To pick on the Roman's we know that they took their hygiene very seriously. They had large and numerous bath-houses through-out rome and their empire. While soap didn't exist (or only the very rich owned it/could use it) they instead used oil as a way of removing dirt. They covered their bodies with it and then scraped it away with a tool known as a Strigil this would also have the benefit of killing or greatly reducing the number of lice on the body. So their genitalia hygiene was likely quiet good and didn't smell or produce odours out of the norm for the time.

Unless they had a sexually transmitted infection[STI].

Most of the documentation out there that we can find isn't about health genitals it's about unhealthy ones! We know things like Lice for instance have been around since more or less the dawn of man. In fact Herodotus [500-424 BC] Describes how Egyptian priests got rid of lice: "The priests shave themselves all over their body every other day, so that no lice or any other foul thing may come to be upon them when they minister to the gods."

Other STI's are mentioned in the bible and early religious text however it isn't until much later that STI's are not only recognized but tracked and treatments are devised (being effective or otherwise).

One of the earliest well documented cases of STI infections/epidemic is from 1494. Syphilis was rapidly spread among French troops besieging Naples. There are some speculations it was transmitted to the French via the Spanish serving the King of France. From there it spread like wild fire across europe and into the new world, killing an estimated 5 million people in Europe.

Gonorrhea aka "the clap" has ties going back to the bible some scholars translate zaavh (female version) and zav (for males) as gonorrhea. Early treatments (as early as 1512) were said to be mercury injections directly into the urinary meatus. Silver proteinate became the treatment of choice from 1897 until antibiotics came on the market in the 1940's.

The "Social Hygiene" movement of the late 19th and early 20th century was a powerful tilt in the world of sexual health and cleanliness. They formed in order to control venereal diseases (now know at STI's) regulate prostitution and share scientific teachings. It is here were many of our common sexual health misconceptions come from (such as women should douche to "Stay clean"). Also they were a driving force behind the pseudoscience of eugenics and forced sterilization of "undesirable" (including those with STI's) was the norm up until the 1930's. This took place not only in Nazi Germany but in America, Canada, and Sweden which didn't stop forced sterilizations until 1976.

I could continue with examples but in order to save you reading I'll finish with this; sexual health and cleanliness is fluid and what's "in" right now might not be "in" later. I'm always amused that people seem to think that anal sex has been "invented" in the last 20 years or so. There are cases going back to Roman times, and heck even early pornography video depicts it.

TL;DR Keeping genitalia "clean" is part natural occurrence and part cultural depending on the era. The real battle was/is with STI's and stopping the epidemic spread.

Interesting reading: History of Syphilis

I have a great answer for this (finally something I can answer!). On my phone right now so just commenting so I can come back and give you a proper /r/askhistorians answer.