Rainbow groaned. “Why you little…” She lunged at him. Spike’s tail wagged harder. She’d taken the bait. He grabbed her torso with his forelegs. She pushed him over and locked her legs around his hips. He began licking her face, over and over, soaking it with spit. She grabbed his mouth with both hands, holding it shut. She wasn’t big, but she was strong. Wiry.

A dog's cock is always hard and doesn't change length. Rather it can move back and forth in the sack. So it protrudes out. It does swell with blood like a human penis but it doesn't get longer. Rather it get's wider. Also when a dog's cock swells the dog itself becomes incredibly passive. This is because dogs don't do foreplay so when the cock swells it's already in the bitch. Spike would likely be able to push through this but it would be a struggle for him.
Also a dog's cock is held hard by a bone that runs along it's length. That means that it feels very different from a human which is just inflated with blood. It truly is rock hard. Also the swelling of the cock is quite pronounced. It usually swells to 4 times the circumference.