You married Doflamingo just because of an arranged marriage, so he won't take you or your status seriously in his eyes. He even once said to you firmly:

"This is just an arranged marriage, so I won't even touch a single piece of you."

But one night, he himself broke his vow. With how he pinned you down on the bed and how he just threw his glasses away and done it under the influence of alcohol roughly with both of you covered in his huge furry coat. In the morning, he woke up and realized what he did last night after seeing that he wasn't wearing anything, he then turned his head to the side and saw you still sleeping soundly because you were exhausted, he groaned in annoyance before speaking

"So I did it.."