He gets a little overzealous

Seeing Erza, who was normally such a dominating presence in the Guild, completely obedient to him, squealing with pleasure in a way not unlike how a puppy looks at it's Master made Natsu feel... Strangely dominant.

Sure, he's been dominating girls since he got the notebook. But this was different. This was Erza. He started to feel that same rush of pleasure he would get from fighting a powerful opponent. A desire to go all out. Full strength.

He reached down, grabbing Erza's hair. She reeled back in pleasure.

"AH!" She yipped "Y-YES MASTER! USE ME AS YOU PLEASE!"

Natsu began pounding into her as hard and fast as he could muster. Slamming his abs into her ass repeatedly as his dick fired in and out of her with immense power.

Erza began to pant and moan as Natsu used her hair as a means to increase his thrusting power. Launching himself back and forth.

"Heh. That's a good girl Erza. Keep taking it!"

"I'M YOURS MASTER!" She boomed, basically unable to even think about anything other than pleasure as she was used like an object for Natsu's desires.

He pulled back harder on her hair as he felt his dick tense, gritting his teeth.

Erza's head looked directly upwards, her eyes rolling back in her head as she let out a squeal.

Natsu blew his load all over her insides, then flopped onto the floor, breathing heavily

"Oh man... That was a workout huh?" He panted...

As Erza turned around to face him, then fell over as well, landing on top of him, barely maintaining her consciousness.