She is faking it look at the position of her tail, its down. If she was doing the nasty you would not be able to see her tail, it would be against his stomach. Also those are two girls. The boys are mud brown and the girls are grey. This is not uncommon for girls to do things like this.
Also in Grad School part of my Experimental Psychology training we talked a lot about sexual behavior in animals, the proff was a dirty old man who taught the sexual behavior of any animal you can name. Want to know how porcupines do it, if you look at a female porcupine there is one small part of her boddy where she does not have quills.

We are the only animal that does the nasty face to face.
All the others use the mounted approach. When a male porcupine taps the hind end of a receptive female she will arch her back and stick her butt in the air, called Lodosis, makes it simple for him to mount her and keeps his head in the air so that he can watch for predators. A female that is not receptive will do one of two things, either ignore him or claw him. Males learn very fast at a very early age how to recongize the difference and they learn that from dad.
Ever wonder why animals have tales, to keep dirt and junk from getting in them.