The inside lining of your mouth: not skin, not mucous membrane? Then what is it?

Is the lining of your mouth just an extension of your exterior skin? I’m pretty sure it’s not mucous membrane, since it’s (AFAIK) not in itself porous to viruses like HIV. What is it?

ETA: And how far does it go down? The back of your throat? Down into the esophagus?

What makes you think it is not mucousmembrane? Mucosa are not generally porous, anyway, it is the blood vessels that HIV gets into.

ETA: I think there is a transition into the throat, but the top ends of the oesophagus and respiratory tract are formed of the same sort of epithelium as the oral mucosa IIRC. Note, this is from staring at slides some 15 years ago, but I am sure someone will be along with more bona fides.

Well, as I said, I had the (I guess mistaken) impression that mucous membranes as found in the nose and the vagina were open pathways for germs to get in, but that didn’t seem to be the case for the mouth.

ETA: Follow up question: what is the surface of your lips made of, then?

Lips, skin, mouth: it’s all different types of epithelium. It seems you’re trying to classify things into a skin/not skin classification system; in reality things are more complicated than that (as usual). Check out the wiki on epithelium.

This has more to do with how relatively well-supplied they are with capillaries than their membrane component, I’d say.
The vagina’s actually pretty hostile to invaders, due to both mucus properties and the activities of symbionts. The thing that makes anal sex riskier than vaginal sex for AIDS is the greater likelihood of tearing & bleeding, after all, which tells me that it’s there that the risk lies. I mean, the point (well, one of the points) of mucus is to be a protective layer preventing things getting in, which is why it contains antibacterial enzymes etc.

in a word, “skin”. Lightly-keratinised stratified squamous epithelium, to be more specific.

The epithelial cells lining the mouth are like the epithelial cells everywhere else in the central tube of the body, in that they are “simply” little bags of cytoplasm with a substantial nucleus. They can be scraped off very easily and if you do that with a toothpick and stain them with iodine, you can see them very clearly under a microscope of anything over about 100x.