Right, because a thirteen year old giving in and getting it over with despite how much she fears the significantly larger, older, warlord she was forcibly married to, and despite the fact that she doesn't want to do it is, that's completely unheard of. Earlier in the chapter, we also get:

As the khal was saddling the horse, Viserys slid close to Dany on her silver, dug his fingers into her leg, and said, "Please him, sweet sister, or I swear, you will see the dragon wake as it has never woken before."

The fear came back to her then, with her brother's words. She felt like a child once more, only thirteen and all alone, not ready for what was about to happen to her.

Which to me, reads as some pretty significant coercion. Also,

Drogo swung off his horse and lifted her down from hers. She felt as fragile as glass in his hands, her limbs as weak as water. She stood there helpless and trembling in her wedding silks while he secured the horses, and when he turned to look at her, she began to cry.

And,

He removed her silks one by one, carefully, while Dany sat unmoving, silent, looking at his eyes. When he bared her small breasts, she could not help herself. She averted her eyes and covered herself with her hands. "No," Drogo said. He pulled her hands away from her breasts, gently but firmly, then lifted her face again to make her look at him. "No," he repeated.

"No," she echoed back at him.

Quite frankly, he did all those 'preparations' to a thirteen year old girl who was behaving as though she very much didn't want to do it. She did help undress him, but she also sat there like a quivering lump and just silently endured his 'foreplay'.

It was more consensual than as depicted in the show, but that's not very hard to do, and just because it's is more gentle does not make it any less rape.