Unless you're one of the unfortunate few who are allergic, which has the potential to be life-threatening, or you're so exceptionally tiny "down there" that the physical size of a dog's penis causes extreme discomfort, allowing a dog to put his penis in your vagina (I'm assuming from the question that you're equipped with one of those) is your basic "no big deal" concept.
Keep in mind that even if you are pretty small, a vagina is designed to stretch, both in length and width, to allow a penis of almost any (reasonable - buzz off, you idiots that want to drool over the fantasy of fucking an elephant, or other simialrly stupid ideas) size to be inserted, and that a dog penis usually enters a vagina - either a dog's, or a woman's - while quite slim, and doesn't expand very much until after it's inside. Which USUALLY means that a small woman can easily accept being penetrated by even a large dog without much, if any, discomfort. The only exception being the knot - As long as he gets deep enough inside you for his knot to swell inside the ring of muscles at the entrance to the vagina, rather than being rammed through that ring, you need only worry about him pulling out while the knot is full size. Even then, it's usually only going to be a few minutes of "Damn! That smarts!" level pain that will pass in short order. A vagina (unlike the anus... more on that later) actually being torn by a "premature withdrawal" while fully erect is *EXTREMELY* unusual - so unusual that I'd rank it as damn near impossible - After all, the vagina can, and routinely does, stretch to a large enough size for a baby to come out - an object that's AT LEAST 5-6 times larger than the largest dog knot I've ever even *HEARD OF*, never mind seen or experienced.
On the other hand, the anus is much "tighter" and less flexible than the vagina - a fully erect knot pulling out of the anus can - not "will", only "can", with too many variables to fully cover in a forum post - cause physical damage because the anus doesn't stretch NEARLY as easily as the opening of the vagina. So if you're going to let Fido "take a drive up the hershey highway", as some enjoy doing, you need to be fairly careful about how large his penis, especially the knot, gets when he's fully erect, and take steps to prevent him from either inserting his knot, or pulling out of you before his knot has deflated enough to pass.
As far as messing up the pH, *ALL* semen is slightly basic, while every vagina I know anything about is slightly acidic. If you've ever allowed a human male to ejaculate inside your vagina, the chances are good that you'll have pretty darn close to the exact same reaction (if you had any at all) if/when you let a dog ejaculate inside you. The main difference being that if/when you let a dog mate with you, he's almost certainly going to inject *MUCH* more semen into you than any ten human males would, and his semen is going to be way thinner - very nearly as thin as plain water or urine - than a human male's semen, which, assuming you're sexually active with humans, you probably already know has a consistency that's much thicker - a lot more like egg whites, or, dare I say it? snot - so unlike with a human male, where any backflow is usually fairly slow and only a small amount, you should be prepared for a fairly serious "gush" when he's finished and withdraws - have a good thick towel within reach, in other words.