I don't think the dog actually has it "in". The cat doesn't seem to be experiencing any discomfort or panic, it just looks annoyed at being held.
Dogs are just as enthusiastic with furniture, doesn't mean they've achieved penetration.
Always possible the cats regular partner is a lion! Who'd even notice a dog after that?
Funniest I've seen for interspecies sex was a guinea pig try to mount a cat.
The cat slept through the whole thing.