I would like to have an explanation for why my cat lies on his side curling his toes and purring like a Ferarri, when I ream him with my nicely lubed and recently showered index finger, besides the simpler explanation that he's a gay or bisexual bottom.
When he is tired of it, he twists his body off and nuzzles my finger, as if to say, "thank you, honey, but that's enough." If I try to do anything more than I already have, he scratches bloody gashes into my hand.
With all felines, "consent" means you are not currently bleeding.
Dogs tend to have a little bit more faith in your good intentions, so you have to use a little more intelligence.
Lube, lube, and lube some more.
A car is not natural, but does that mean you want to walk every day to get to work? Without using sidewalks or thoroughfares in order to get there but instead stumbling the whole way through brambles and poison ivy?
In nature, animals do what it is convenient and rational to do. If humans invented it, then animals use it if they like it.
Coyotes love interstate highway medians and those broad, spacious shoulders that are often the size of football fields. Airports have a huge amount of open space, and they are Coyote Heaven.
"Natural behavior" is what seems to be the most rational possible behavior, which enhances your level of comfort and your chances for survival. Sometimes, what SEEMS to be rational is not really all that rational, such as coyotes camping out on airfields, but it is profoundly natural for them to do what seems like a good idea at the time.
If you try Astroglide with your girl even once during her third heat, then I can almost perfectly guarantee that you will enjoy it a lot, and you have the assurance of many experienced people here that it's also a wise idea.
Your dog will go along with almost anything that you seem to think is fun because she is a dog: she is evolved to assume that anything her human wants to do is, somehow, fun. As a human, though, you might want to think carefully about your definition of fun. She is going to love you, regardless, but you will do her a better service and probably provide a more fun experience if you are both clean and properly lubricated.