I am not knowledgeable enough on historical oral hygiene practices to answer the first part of your question.

However, I can answer your second question:

Has kissing always been a part of people's love lives?

Well, prior to written history, we have a hard time concluding whether or not people kissed. There are various hypotheses for how kissing developed, ranging from "feeding" hypotheses, akin to canines and birds who pass food to juveniles through their mouths, to hypotheses which imply that kissing is a way of exchanging olfactory information. It's a very interesting anthropological discussion.

But from a HISTORICAL perspective, kissing is mentioned in writing about 5,000 years ago, in Sumerian poetry: (Kramer, Samuel Noah (1981). History Begins at Sumer (3rd revised. ed.). Philadelphia: University of Pennsylvania Press., http://etcsl.orinst.ox.ac.uk/cgi-bin/etcsl.cgi?text=t.4.08.04#)

Later examples from Egypt also mention kissing, and are fairly specific: http://www.love-poetry-of-the-world.com/Egyptian-Love-Poetry-Poem2.html, http://themagentahornet.com/ancient-poetry1.html

So I can't tell you about how people felt about kissing due to oral hygiene, but people were definitely kissing in a romantic context since almost the beginning of recorded history.

The "Jests of Hierocles" has a whole section related to bad breath and it references the issues it caused for romance. I know "AskHistorians" isn't normally a venue for jokes, but it seems apropos given the context...

#234: "A person with offensive breath asked his wife, 'Why do you hate me?' She replied saying, 'Because you love me.' "

Available on Google Books: http://books.google.com/books?id=OtMb_pzRnOoC&pg=PA15&lpg=PA15&dq=jests+of+hierocles&source=bl&ots=jCnIp0LSmP&sig=MmNtKc_OVkvrlzjJbL7EWkMqNr4&hl=en&sa=X&ei=W1-PU6-AGo_6oASJs4LoBQ&ved=0CEUQ6AEwBA#v=onepage&q=breath&f=false

I'd like to try to answer this, but the way you've worded this questions makes it difficult.

Kissing has been around quite a while - some early evidence we have for kissing dates back to around 1500 B.C.E. from The Four Vedic Sanskrit Texts (The Vedas). There's no mention of the word “kiss,” (or the Sanskrit version of it, rather) but there are references to “licking,” and “drinking moisture of the lips.” By the third century C.E. we have the Vatsyayana Kamasutra (better known as the Kama Sutra), which includes a chapter describing how to kiss.

India wasn't the only part of the world kissing that far back (e.g. kissing is mentioned in Homer's epics), but it's a convenient example.

Some experts - I'm citing Vaughn Bryant, an anthropologist at Texas A&M - have speculated that kissing came from sniffing. Basically, that there was a lot of sniffing / sniffing greetings going on, and "at some point, they slipped and ended up on the lips, and they thought that was a lot better." (Bryant). Another good source for this kissing / sniffing theory is The Science of Kissing, by Sheril Kirshenbaum.

When you say "oral hygiene" and "contemporary methods" is where I lose you. I'm not sure what you mean by these terms - people have been cleaning our teeth as far back as written history goes. Assyrian cuneiform medical texts from 3000 B.C.E. mention teeth-cleaning procedures and toothpicks from roughly the same time have been found in Iraq (Mesopotamia). Greek writings note that Aesculapius - the Greek god of medicine - advocated for oral hygiene.

So I suppose my problem is that, without knowing what you consider "oral hygiene" or "contemporary methods", your question can't be answered. Based on what I remember and what I've read, at least, there's no record that a lack of oral hygiene was a deterrent to kissing.

So I guess, no?

Edit: Poorly researched sidenote - I remember reading that body odor being considered unpleasant is a relatively recent phenomenon, associated with a lack of cleanliness, and that for a long time people liked the smell of BO. Perhaps hand in hand with that is not minding bad breath quite as much.

Anyway, this isn't really my area but I hate to see this thread look so raggedy. Best answer I've got for you, sorry I cant' do better.

It's important to remember that oral hygiene wasn't always lacking in the ancient world. It depends very much on which population and which time period you're talking about.

For example, Indians have been using twigs from the neem tree to brush their teeth for thousands of years (and still do today). The way it works is that they take a twig and chew on it for a few seconds until the fibers separate, and then it becomes a toothbrush. They then use this to brush their teeth.

The mechanical action of brushing, as well as antimicrobials present in the neem tree, are very effective at combating tooth decay. In fact, studies have found no statistically significant difference between the incidence of plaque, cavities and gingivitis using this method, compared to a modern toothbrush and toothpaste.

Similarly, in many Arab countries the twig of the Arak tree (called a miswak) is used as a toothbrush. Studies have also shown that using a miswak is just as effective as using a modern toothbrush/paste.

It's also good to remember that the incidence of tooth and gum diseases is very related to one's diet. Specifically, diets that are high in sugars and simple starches, or carbonated drinks, predispose to tooth decay and gum disease. Such diets were not widely available in the distant past, so the incidence of these diseases was also low. There are studies on populations living thousands of years ago that show many ancient populations had exceptionally healthy teeth.

Isn't it worth mentioning that before the advent of processed foods, oral hygiene was not as necessary as it is today? For example, tribal people still have very healthy teeth. Or is this a misconception?

I can't answer for kissing on the mouth, but I do know that, tragically, oral sex was considered taboo in Ancient Greece and Rome due to a fear of "pollution" - and not just limited to worries of hygiene. In Greco-Roman society, where there was so much interaction by way of conversation, kisses on the cheeks and mouth, and communal eating and drinking, there was a fear that oral sex could cause some sort of moral pollution, with a physical manifestation comparable to a curse. Additionally, it was thought that oral sex was a form of penetration, so a man who gave head would probably be treated rather similarly to a man who enjoyed getting pegged in the modern day (sort of).

As a result, there's a lot of poetry by more salacious Roman poets on the subject - Martial has over sixty poems that mention it, and one of the Latin schoolboy's favourite poems, Catullus 16, is bookended by the phrase, "I'm going to bum you both and make you blow me." Also, there's a graffito from Pompeii which rather succinctly sums up Roman attitudes to cunnilingus:

III.5.3 (on the wall in the street); 8898: Theophilus, don’t perform oral sex on girls against the city wall like a dog

Think on that next time you watch that scene with Jon Snow and Ygritte in the cave.

If I remember correctly, in Chaucer's "Miller's Tale," Absalon sweetens his breath with cardamon and licorice in hopes of stealing a kiss from the adulterous Alison. I think this could possibly suggest some awareness of bad breath in the 1300s when the Canterbury Tales are thought to have been written.

Well, I'm not a historian and so I'm not going to bother with the kissing issue, although I notice that no-one had discussed the nerve-density and sensitivity of the mouth which qualifies as a specific erogenous zone in a biological sense - not a social or cultural context but in objective sensitivity.

As for oral hygiene, wouldn't there have been differences in diet so that people might not have had the same issues we have today?

It just seems to me like you can't simply assume people had bad oral hygiene just because it was long ago. I'd like to see proof that it was the case, or was wide-spread, before assuming that poor oral hygiene was widespread. Seriously, I'd like to ask where the "people were excessively dirty" thing came from regarding historical societies.